HARVARD UNIVERSITY
Library of the Museum of
Comparative Zoology
ThcWlsonBulktm
PUBLISHED BY THE WILSON ORNITHOLOGICAL SOCIETY
VOLUME 89
1977
QUARTERLY
editor: JEROME A. JACKSON
REVIEW editor: ROBERT RAIKOW
COLOR PLATE EDITOR: WILLIAM A. LUNK
EDITORIAL ASSISTANTS: BONNIE J. TURNER
PATRICIA RAMEY WAYNE C. WEBER BETTE J. SCHARDIEN MARTHA HAYS
President — Douglas A. James, Department of Zoology, University of Arkansas, Fayetteville, Arkansas 72703.
First Vice-President — George A. Hall, Department of Chemistry, West Virginia Univer- sity, Morgantown, W. Va. 26506.
Second Vice-President — Abbot S. Gaunt, Department of Zoology, Ohio State University, Columbus, Ohio 43210.
Editor — Jerome A. Jackson, Department of Zoology, P.O. Drawer Z, Mississippi State Uni- versity, Mississippi State, Mississippi 39762.
Secretary' — James Tate, Jr., P.O. Box 2043, Denver, Colorado 80201.
Treasurer — Ernest E. Hoover, 1044 Webster St., N.W., Grand Rapids, Michigan 49504.
Elected Council Members — Sidney A. Gauthreaux, Jr. (term expires 1978) ; James R. Karr (term expires 1979) ; Clait E. Braun (term expires 1980).
DATES OF ISSUE OF VOLUME 89
OF THE WILSON BULLETIN
AO. 1 — 7 April 1977 NO. 2 — 27 June 1977 NO. 3 — 30 September 1977 NO. 4 — 27 December 1977
CONTENTS OF VOLUME 89
NUMBER 1
NOTES ON THE BIOLOGY AND POPULATION STATUS OF THE MONKEY-EATING EAGLE OF THE PHILIPPINES Robert S. Kennedy
COWBIRD PARASITISM AND EGG RECOGNITION OF THE NORTHERN ORIOLE . Stephen I. Rothstein
OBSERVATIONS ON THE RED-NECKED GREBE NESTING IN MICHIGAN Michael L. Chamberlin
GROWTH AND DEVELOPMENT OF THE PLAIN CHACHALACA IN SOUTH TEXAS Wayne R. Marion
SOCIAL DOMINANCE IN WINTER FLOCKS OF CASSIN’s FINCH Fred B. Samson
INTER-BROOD MOVEMENTS OF JUVENILE SPRUCE GROUSE Daniel M. Keppie
BREEDING BIOLOGY OF YEAR-OLD AND OLDER FEMALE RED-WINGED AND YELLOW-HEADED
BLACKBIRDS Richard D. Crawford
FOOD HABITS OF OLDSQUAWS WINTERING ON LAKE MICHIGAN
Steven R. Peterson and Robert S. Ellarson
SEXUAL DIMORPHISM IN THE WHITE IBIS James A. Kushlan
EGGSHELL THICKNESS VARIABILITY IN THE WHITE-FACED IBIS David E. Capen
CHARACTERISTICS OF A WINTERING POPULATION OF WHITE-TAILED PTARMIGAN IN COLORADO
Richard W. Hoffman and Clait E. Braun
BREEDING DENSITIES AND MIGRATION PERIODS OF COMMON SNIPE IN COLORADO
Bruce R. Johnson and Ronald A. Ryder
PRINCIPAL COMPONENT ANALYSIS OF WOODPECKER NESTING HABITAT
Richard N. Conner and Curtis S. Adkisson
PHENETIC ANALYSIS OF THE SUBFAMILY CARDINALINAE USING EXTERNAL AND SKELETAL CHARACTERS Jenna J. Hellack and Gary D. Schnell
GENERAL NOTES
WHY OSPREYS HOVER Thonias C. Grubb, Jr.
STORAGE OF PINON NUTS BY THE ACORN WOODPECKER IN NEW MEXICO
Peter B. Stacey and Roxana Jansma
FLOCKING AND FORAGING IN THE SCARLET-RUMPED TANAGER David J. Moriarty
YELLOW WARBLER NEST USED BY A LEAST FLYCATCHER J. Paul GoOSSen
PINTAIL REPRODUCTION HAMPERED BY SNOWFALL AND AGRICULTURE Gary L. KrapU
TICKS AS A FACTOR IN THE 1975 NESTING FAILURE OF TEXAS BROWN PELICANS
Kirke A. King, David R. Blankinship, Richard T. Paul, and Robin C. A. Rice
PRAIRIE WARBLER FEEDS FROM SPIDER WEB John F. DoUglaSS
NOTES ON THE HUMMINGBIRDS OF MONTEVERDE, CORDILLERA DE TILARAN, COSTA RICA
Peter Feinsinger
NEST-SITE DIFFERENCES BETWEEN RED-HEADED AND RED-BELLIED WOODPECKERS IN SOUTH
CAROLINA Lawrence Kilham
GROUND FORAGING AND RAPID MOLT IN THE CHUCK-WILL’s WIDOW
Sievert Rohwer and James Butler
FEEDING RESPONSES OF FALL MIGRANTS TO PROLONGED INCLEMENT WEATHER
Elliot J. Tramer and Elora E. Tramer
S ’THBOUND MIGRATION OF SHOREBIRDS FROM THE GULF OF ST. LAWRENCE
Raymond McNeil and Jean Burton
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33
47
57
67
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81
92
99
107
116
122
130
149
150
151
153
154
157
158
159
164
165
166
167
FLOCKING AND FORAGING BEHAVIOR OF BROW^N JAYS IN NORTHEASTERN MEXICO
Michael L. Morrison and R. Douglas Slack 171
DO MORE BIRDS PRODUCE FEWER YOUNG? A COMMENT ON MAYFIELD’s MEASURE OF NEST
SUCCESS Richard F. Green 173
MORTALITY OF NESTLING MISSISSIPPI KITES BY ANTS James W . Parker 176
ORNITHOLOGICAL NEWS 177
ORNITHOLOGICAL LITERATURE 179
NUMBER 2
THE BREEDING BIOLOGY OF THE OAHU ‘elepaio Sheila Conant 193
LIFE HISTORY FACTORS AFFECTING THE INTRINSIC RATE OF NATURAL INCREASE OF BIRDS OF THE
DECIDUOUS FOREST BIOME Richard Brewer and Lynda Swander 211
NESTING HABITAT OF COMMON RAVENS IN VIRGINIA Robert G. Hooper 233
GROWTH AND DEVELOPMENT OF KNOWN-AGE RING-BILLED GULL EMBRYOS
John P. Ryder and Lynn Somppi 243
HABITAT PARTITIONING IN A COMMUNITY OF PASSERINE BIRDS — Robert C. Whitmore 253
BREEDING DISPLAYS OF THE LOUISIANA HERON James A. Rodgers, Jr. 266
BREEDING BIOLOGY OF CLIFF SWALLOWS IN VIRGINIA Gilbert S. Grant and Thomas L. Quay 286
WEATHER INFLUENCES ON NOCTURNAL BIRD MORTALITY AT A NORTH DAKOTA TOWER
Michael Avery, Paul F. Springer, and J. Frank Cassel 291
BREEDING BIOLOGY OF HOUSE SPARROWS IN NORTH MISSISSIPPI James N. Sappington 300
NESTING BEHAMOR OF YELLOW-BELLIED sApsucKERs Lawrence KUham 310
GENERAL NOTES
SEX DIFFERENCES IN ALARM RESPONSES OF WINTERING EVENING GROSBEAKS
Martha Hatch Ralph 325
NESTING OF TURKEY AND BLACK VULTURES IN PANAMA Laurie A. McHargUC 328
FULVOUS WHISTLING DUCK POPULATIONS IN TEXAS AND LOUISIANA
Edward L. Flickinger, David S. Lobpries, and Hugh A. Bateman 329
SLIPPER SHELLS, A MAJOR FOOD ITEM FOR WHITE-WINGED SCOTERS James G. Hoff 331
EGG MOVEMENT BY A FEMALE GADWALL BETWEEN NEST BOWLS
Robert F. Johnson, Jr. and Leo M. Kirsch 331
FOODS OF WESTERN CLAPPER RAILS R. D. Ohmart and R. E. Tomlinson 332
AGGRESSION IN FORAGING MIGRANT SEMIPALMATED SANDPIPERS
Brian A. Harrington and Sarah Groves 336
HERRING GULL EATING BAYBERRY Roger F. Pasquier 338
THE LESSER ANTILLEAN BULLFINCH IN THE VIRGIN ISLANDS
Herbert A. Raffaele and Daniel Roby 338
FORAGING BEHAVIOR OF THE WHITE IBIS James A. Kushlan 342
BIRDS OF FIVE FAMILIES FEEDING FROM SPIDER WEBS
Robert B. Waide and Jack P. Hailman 345
WINTER NEST MICROCLIMATE OF MONK PARAKEETS
Donald F. Caccamise and W'esley W. W eathers 346
SNAKE PREDATION ON bell’s vireo NESTLINGS Calvin L. Gink 349
CROW PREDATION ON BLACK-CROWNED NIGHT HERON EGGS
Joanna Burger and D. Caldwell Hahn 350
ORNITHOLOGICAL LITERATURE
CONSERVATION COMMITTEE REPORT
(Falconry: Effects on Raptor Populations and Management in North America)
ORNITHOLOGICAL NEWS
NUMBER 3
NESTING HABITAT OF BACHMAN’s WARBLER — A REVIEW
Robert G. Hooper and Paul B. Hamel
MORPHOLOGICAL VARIATION IN NORTH AMERICAN PINE GROSBEAKS Curtis S. AdkisSOn
BREEDING SUCCESS AND NEST SITE CHARACTERISTICS OF THE RED-WINGED BLACKBIRD
Donald F. Caccamise
FORAGING BEHAVIOR OF THE EASTERN BLUEBIRD Benedict C. Pinkowski
COMPARATIVE FEEDING BEHAVIOR OF IMMATURE AND ADULT HERRING GULLS
Nicolaas A. M. Verbeek
COMPARATIVE MORTALITY OF BIRDS AT TELEVISION TOWERS IN CENTRAL ILLINOIS
James W. Seets and H. David Bohlen
EFFECT OF FLOCK SIZE ON FORAGING ACTIVITY IN WINTERING SANDERLINGS
James Silliman, G. Scott Mills, and Stephen Alden
ACTIVITY PATTERNS OF FEMALE RUFFED GROUSE DURING THE BREEDING SEASON
Stephen J. Maxson
WEIGHTS AND FAT CONDITION OF SOME MIGRANT WARBLERS IN JAMAICA
A. W. Diamond, P. Lack, and R. W. Smith
GENERAL NOTES
WING MOLT OF THE KiTTLiTz’s MURRELET Spencer G. Sealy
INCIDENCE OF RUNT EGGS IN THE CANADA GOOSE AND SEMIPALMATED SANDPIPER
T. H. Manning and Brenda Carter
LATE FLEDGING DATE FOR HARRIS’ HAWK Eleanor L. Radke and John Klimosewski
THE SPATIAL DISTRIBUTION OF WINTERING BLACK-BELLIED PLOVERS
Christopher H. Stinson
PREDATION AND DISPERSION OF HERRING GULL NESTS
Gary W . Shugart and William C. Scharf
EGG QUALITY IN RELATION TO NEST LOCATION IN RING-BILLED GULLS
John P. Ryder, Donald E. Orr, and Ghomi H. Saedi
ROOF-NESTING BY COMMON TERNS Anne E. MacFarlane
RAPID CHICK SEPARATION IN WHIP-POOR-WILLS Eric L. Dyer
AN INTRASPECIFIC MORTAL ATTACK Vera Lee Grubbs
RUFOUS-SIDED TOWHEES MIMICKING CAROLINA WREN AND FIELD SPARROW
Donald J. Borror
HEAT LOSS FROM THE NEST OF THE HAWAIIAN HONEYCREEPER, “aMAKIHi”
G. C. Whittow and A. J. Berger
SPREAD OF THE GREAT-TAILED GRACKLES IN SOUTHWESTERN LOUISIANA
H. Douglas Pratt, Brent Ortego, and Harland D. Guillory
POPLAR LEAF-STEM GALL INSECTS AS FOOD FOR WARBLERS AND WOODPECKERS
Sally Hoyt Spofford
PERCH HEIGHT PREFERENCES OF GRASSLAND BIRDS Keith G. Harrison
SPECIAL REVIEW — JOHN OSTROM's STUDIES ON ARCHAEOPTERYX, THE ORIGIN OF BIRDS, AND THE EVOLUTION OF AVIAN FLIGHT Joel CracraH
ORNITHOLOGICAL LITERATURE
ORNITHOLOGICAL NEWS
352
360
370
373
380
396
404
415
422
434
439
456
467
469
469
470
472
473
475
476
477
477
480
483
485
485
488
492
503
505
THE president’s PAGE
PROCEEDINGS OF THE FIFTY-EIGHTH ANNUAL MEETING 506
SUGGESTIONS TO AUTHORS 520
NUMBER 4
THE LESSER PRAIRIE CHICKEN’s INFLATABLE NECK SACS George Miksch SuttOtl 521
NESTING HABITAT OF CANADA GEESE IN SOUTHEASTERN MICHIGAN
Richard M. Kaminski and Harold H. Prince 523
RESIDUES OF ENVIRONMENTAL POLLUTANTS AND SHELL THINNING IN MERGANSER EGGS
Donald H. White and Eugene Cromartie 532
BREEDING BIRD SURVEY COUNTS AS RELATED TO HABITAT AND DATE
Wayne C. Weber and John B. Theberge 543
ANALYSIS OF MATERIALS IN CLIFF AND BARN SWALLOW NESTS: RELATIONSHIP BETWEEN
MUD SELECTION AND NEST ARCHITECTURE Delbert L. Kilgore, Jr. and Kathy L. Knudsen 562
PRODUCTION AND SURVIVAL OF THE VERDIN George T. Austin 572
MALE BEHAVIOR AND FEMALE RECRUITMENT IN THE RED-WINGED BLACKBIRD
Patrick J. W eatherhead and Raleigh J. Robertson 583
REDUCTION OF COURTSHIP BEHAVIOR INDUCED BY DDE IN MALE RINGED TURTLE DOVES
M. A. Haegele and Rick H. Hudson 593 MOVEMENTS OF THE GREAT-TAILED GRACKLE IN TEXAS .. Keith A. Arnold and Lcon J. Folse, Jr. 602
GENERAL NOTES
TROPICAL SCREECH OWL NEST DEFENSE BEHAVIOR AND NESTLING GROWTH RATE
Betsy Trent Thomas 609
THREE MORE NEW SPECIMEN RECORDS FOR GUATEMALA Robert W. Dickermau 612
FEEDING BEHAVIOR OF TWO HUMMINGBIRDS IN A COSTA RICAN MONTANE FOREST
Barbara K. Snow 613
BLACK-LEGGED KITTIWAKES NESTING ON SNOWBANK
George L. Hunt, Jr. and Max C. Thompson 616
EVIDENCE OF DOUBLE BROODING BY AMERICAN KESTRELS IN THE COLORADO HIGH PLAINS
Dale W. Stahlecker and Herman J. Griese 618
FURTHER COMMENTS ON SEXUAL SIZE DIMORPHISM IN BIRDS D. Amadon 619
RESPONSE OF INCUBATING BLACK-BELLIED WHISTLING-DUCKS TO LOSS OF MATES
Richard E. McCamant and Eric G. Bolen 621
LATE PLEISTOCENE WILLIAMSON’s SAPSUCKER FROM WYOMING
Michael Wilson and Amadeo M. Rea 622
AMERICAN KESTREL REJECTS CAPTURED SPADEFOOT TOAD G. ScOtt Mills 623
WINTER DISTRIBUTION OF RED-TAILED HAWKS IN CENTRAL NEW YORK STATE Jonathan Bart 623
OSPREY CATCHES VOLE Noble S. Proctor 625
PATTERNS OF FEEDING FIELD SPARROW YOUNG Louis B. Best 625
AVIAN BONE PATHOLOGIES FROM ARIKARA SITES IN SOUTH DAKOTA Paul W . Parmalee 628
NEST RECIPROCITY IN EASTERN PHOEBES AND BARN SWALLOWS Hamion P. Weeks, Jr. 632
ORNITHOLOGICAL LITERATURE 636
ORNITHOLOGICAL NEWS 655
REQUESTS FOR ASSISTANCE 571, 635
INDEX TO VOLUME 89
657
The Wilson Bulletin
PUBLISHED BY THE WILSON ORNITHOLOGICAL SOCIETY
VOL. 89, NO. 1 MARCH 1977
ARY
APR 2 01977
t“-i A R V A K
U I V i£RS i X Y,
The Wilson Ornithological Society Founded December 3, 1888
Named after ALEXANDER WILSON, the first American Ornithologist.
President — Andrew J. Berger, Department of Zoology, University of Hawaii, Honolulu, Hawaii 96822.
First Vice-President — Douglas A. James, Department of Zoology, University of Arkansas, Fayetteville, Arkansas 72703.
Second Vice-President — George A. Hall, Department of Chemistry, West Virginia Uni- versity, Morgantown, W. Va. 26506.
Editor — Jerome A. Jackson, Department of Zoology, P.O. Drawer Z, Mississippi State Uni- versity, Mississippi State, Mississippi 39762
Secretary — James Tate, Jr., P.O. Box 2043, Denver, Colorado 80201.
Treasurer — Ernest E. Hoover, 1044 Webster St., N.W., Grand Rapids, Michigan 49504.
Elected Council Members — Helmut C. Mueller (term expires 1977) ; Abbott S. Gaunt (term expires 1978) ; James R. Karr (term expires 1979).
Membership dues per calendar year are: Active, $10.00; Sustaining, $15.00;
Life memberships, $200 (payable in four installments).
The Wilson Bulletin is sent to all members not in arrears for dues.
The Josselyn Van Tyne Memorial Library The Josselyn Van Tyne Memorial Library of the Wilson Ornithological Society, housed in the University of Michigan Museum of Zoology, was established in concurrence with the University of Michigan in 1930. Until 1947 tlie Library was maintained entirely by gifts and bequests of books, reprints, and ornithological magazines from members and friends of the Society. Now two members have generously established a fund for the purchase of new books; members and friends are invited to maintain the fund by regular contribution, thus making available to all Society members the more important new books on ornithology and related subjects. The fund will be administered by the Library Committee, which will be happy to receive suggestions on the choice of new books to be added to the Library. William A. Lunk. University Museums, University of Michi- gan, is Chairman of the Committee. Tlie Library currently receives 104 periodicals as gifts and in exchange for The Wilson Bulletin. With the usual exception of rare books, any item in the Library may be borrowed by members of the Society and will be sent prepaid (by the University of Michigan) to any address in the United States, its possessions, or Canada. Return postage is paid by the borrower. Inquiries and requests by borrowers, as well as gifts of books, pamphlets, reprints, and magazines, should be addressed to “The Josselyn Van Tyne Memorial Library, University of Michigan Museum of Zoology, Ann Arbor, Michigan.” Contributions to the New Book Fund should be sent to the Treasurer (small sums in stamps are acceptable). A complete index of the Library’s holdings was printed in the September 1952 issue of The Wilson Bulletin and newly acquired books are listed periodically.
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The official organ of the Wilson Ornithological Society, published quarterly, in March, June, September, and December. The subscription price, both in the United States and elsewhere, is S15.00 per year. Single C(>i)ies, S4.00. Subscriptions, changes of address and claims for undelivered copies should be sent to the Treasurer. Most back issues of tlie Bulletin are available and may be ordered from the Treasurer. Special prices will be quoted for <|uantity orders.
All articles and communications for publications, books and publications for reviews should be addressed to the Editor. Exchanges should be addressed to The Josselyn Van Tyne Memorial Library, Museum of Zoology, Ann Arbor, Michigan. Known office of publication: Department of Zoology, Mississippi State University,
Mississippi State, Mississippi 37962.
Second class postage paid at Mississippi State, Mississippi and at additional mailing office.
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THE WILSON BULLETIN
A QUARTERLY MAGAZINE OF ORNITHOLOGY Published by the Wilson Ornithological Society
VoL. 89, No. 1 March 1977 Pages 1-192
NOTES ON THE BIOLOGY AND POPULATION STATUS OF THE MONKEY-EATING EAGLE OF THE PHILIPPINES
Robert S. Kennedy
The Monkey-eating Eagle \ Pithecophaga jejferyi^ see Frontispiece) is a huge forest raptor endemic to the Philippines. After its discovery in 1896 on the island of Samar ( Ogilvie Grant 1897b ) , it was considered so rare that nearly every specimen obtained before 1940 prompted a published account. Gonzales ( 1968) provided the first life history data in his 10-month study of a nesting pair in the province of Davao del Sur, on the island of Mindanao.
Field censuses of this endangered species in 1969 on Mindanao, where the species is most abundant, produced estimates of 40 ( Alvarez 1970 ) and 36 (Gonzales 1971). Rahor (1971) estimated the numbers in 1970 at 50 to 60 birds.
During a study of this species on Mindanao from August 1972 to April 1973, I assisted the Philippine Research, Parks, Range, and Wildlife Division of the Bureau of Forest Development in efforts to conserve this eagle. I col- lected data on the behavior of wild eagles and on their numbers and distribu- tion. Here I report on my findings and include a summary of the former and present status of the species on other islands where it has been recorded.
STUDY AREA AND METHODS
I studied activities of a pair of Monkey-eating Eagles at Tudaya Falls in Mt. Apo Na- tional Park on Mindanao. The area is dissected hy a series of deep ravines carved by swift mountain rivers. Elevation ranges from 700 to 1200 m. Because of the close prox- imity to the equator, the time for sunrise and sunset varied little over the year. A canyon below Tudaya Falls was the primary study area. It was ca. 400 m wide, 100 to 200 m deep, and 2 km long (Fig. 1). On the ridges surrounding the canyon, Bagoho natives have cleared some of the virgin forest (see Fig. 2). I spent 8 full days and 14 one-half days from September to March (153 hours of observation) at 3 lookouts overlooking the canyon, the choice of which depended on the location of the eagles at the time.
With personnel from the Philippine Parks stationed in Davao City and Zamboanga City, I traveled to 10 provinces of Mindanao: Lanao del Norte, Missamis Occidental, Zamboanga del Norte, Zamboanga del Sur, Davao City, Davao del Norte, Davao Oriental,
1
2
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
Fig. L View of canyon in tlie Tudaya Falls study area. IMioto taken from Lookout ^I facing tin* southeast.
Davao del Sur, North Cotahato, and South Cotahato. Coverage was greatest in the last 6 provinces. Keeords of eagles killed, sighted, or eaj)tured since 1970 were collected from local residents and wer<‘ ae(“»*pted or rejected on the basis of the information provided. Fersonnel from logging companies and natives provided the most reports.
Aerial surveys over 12 of the 17 provinces of Mindanao facilitated i)lotting the dis- tribution of Monkey-eating Eagle habitat on .joint Operation (Graphic (AIK) Maps, Series 1501 AIK, 1:250,000, current through 1909. Observations from the ground contributed additional data for habitat j)lotting. Frovinces surveyt'd by air in their entirety were Davao City, Davao del .'*'ur, Lanao del l^ur, and Missamis Occidental. Partially surveyed were Bukidnon, Davao del Norte. Davao Oriental. Lanao del Norte. North Cotahato. South Cotahato. Zamboanga del Norte, and Zamboanga del Sur. For other areas, the extent of the habitat was estimated from the topography and the density of human habitations, as j)rinted on the air maps. Areas of (piestionahle human density were plotted with the 30(K) foot contour line as the {)erimeter of the eagle’s habitat. By use of squared graph paper, the area of potential (‘agle habitat was determined from the air maps. The method does not account for the inereast' in surface area due to variation in altitude.
IJEHAVIOR OF WILD EAGLES
Hunting, techniques. — Monkey-eating Eagles hunt lioth singly and in pairs. I (lid not see a pair of eagles hunt together, hut several natives and loggers
Kennedy • BIOLOGY OF MONKEY-EATING EAGLE
3
DAVAO CITY i
DAVAO DEL SUR
Fig. 2. Map of Tudaya Falls showing the distribution of cleared land (shaded area), forests (unshaded area), sightings of eagles (dots), and lookouts (triangles).
reported that pairs of eagles course through the forest looking for groups of monkeys. An engineer with the Misamis Lumber Co., stated that one eagle would distract the monkey, which would then he captured from behind by the other bird. He reported that, after the kill, the eagle covered the prey with its wings and then gutted and skinned the animal. Gonzales ( 1968 j suggests that eagles are more successful when hunting in pairs than when hunting alone, because of the wariness of monkeys and the defense of the family unit by a lead male.
Thirty of 38 observations at Tudaya Falls, and several elsewhere on Min- danao, were of eagles on the hunt. Though I never observed a complete hunting sequence, because of the bird’s sudden appearance and disappearance in the forest, a general 3 part hunting pattern can he reconstructed as follows (Fig. 3) :
Part 1 — Preparatory Period : On 28 September I watched an eagle perched on the lower branch of a tree above the near-vertical cliffs across the canyon from Lookout #3 (Fig. 2). There it called from 13:00 to 13:30. It then be- came increasingly alert to the sounds and movements in the canyon below. The usual position of a perched Monkey-eating Eagle is vertical, and, from a distance, the white breast and belly of a bird in this stance closely resemble the light-colored bark of the trunks and main branches of many Philippine
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THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
Fig. 3. Schematic representation of a liunting sequence, consisting of three parts; Part 1 — a preparatory jicriod; Part 2 — the act of hunting within the forest canopy; Part 3 — return to a starting point.
trees. This may lie a form of cryptic coloration. Because I observed the preparatory period only once, I do not know if the calling activity is charac- teristic of all hunting sequences.
Part 2 — dlie Act of Hunting: At 13:35, the eagle dropped from its perch and without flapping, glided about 75 in at a downward diagonal, along the canyon wall, to a resting iilace in a small tree growing out of the canyon wall. Be- cause of the minimal vegetation growth, I could see the bird clearly. It re- mained there for 5 min surveying the surrounding trees. When the eagle seemingly perceived movements, it would shift its head to the right and left of the body axis, fretjuently twisting its head upside down. It made 6 more short flights liefore it disappeared at 14:35. In flights ranging from 75 to 125 m, the bird moved toward the floor of the canyon. The short direct flights from one perch to another usually within the forest canopy (Fig. 3) are the most common hunting techniiiue used by the eagles when they work down a mountain slope or along a heavily forested ridge. I noted one variation from the pattern: on steep slopes, instead of gliding directly to a perch, the eagle would frequently drift out of the forest canopy aw ay from the slope, circle one or more times, and then return to another perch. During these flights, the birds searched the forest around them, apparently for prey or for a suitable
Kennedy • BIOLOGY OF MONKEY-EATING EAC;LE
5
perch. J. Hamlet ( pers. comm. ) described the eagle in pursuit of prey as having direct, flapping flight.
Part 3 — Return to a Starting Point: If the eagle failed to capture prey, it would return to a higher elevation to initiate a new hunt. From Lookout #2, on 13 February 1973, I saw a Monkey-eating Eagle perched on the lower branch of a large tree at the forest edge, 400 m west of my position. At 09:45, it left the perch and glided parallel to the canyon, heading south for approxi- mately 500 m, with minimal loss of altitude. Then it began to circle slowly, gaining altitude. When it attained a height of 300 to 450 m above its initial ele- vation, the eagle glided directly north for 2 to 3 km, disappearing into the for- est higher up the mountain at 10:05. This same eagle (which had 2 left pri- maries missing) reappeared at the forest edge at 13:45 and repeated a similar sequence. 1 saw 3 eagles elsewhere on Mindanao performing this part of the hunting technique with little deviation from the pattern. Elevations attained varied from 150 to 700 m above the start of the spiral, and the distance glided varied from 400 m to 3 km. Part 1 lasted 35 min. Part 2, 60 min, and Part 3, 20 min.
If one assumes that the eagle used to exemplify Part 3 did not engage in any other activities besides hunting from the time it was first seen at 09:45 to the time it reappeared at 13:45, then this hunting cycle lasted 4 h. In another case, the cycle lasted 2 h 55 min. The times for the 3 hunts average 2 h 56 min.
Figure 4 indicates 2 peak periods in the day when eagles are likely to be seen. During these periods, the birds often emerge from the forest and fly to another location on the mountain, as described earlier. For the morning peak, 8 of 11 and 7 of 8 sightings made at 09:00 and 10:00, respectively, were of eagles ostensibly hunting. In the afternoon, the sightings at 13:00 and 14:00 were of eagles hunting. The peaks for calling (Fig. 4) suggest that vocalizations occur in Part 1 of the hunting cycle.
I observed the result of a successful hunt at Tudaya on the morning of 16 February 1973. While at Lookout #1, I heard an eagle calling at 08:20 from the side of the canyon. It was apparently perched in a tree, and I did not see it until 15 min later, when it stopped calling, flew over the canyon, circled once, then glided 150 m west up Parak Creek and landed in a small tree. I noticed a monkey in the eagle’s talons when the bird flew past me. At the perch, the eagle resumed calling but did not mantle the prey or attempt to eat it. At 08:45, the eagle left its perch, glided west about 500 m, and disappeared into the forest. The manner in which it traveled from one resting place to another was similar to Part 2 of the hunting cycle.
Flight. — Brown and Amadon (1968) state that Monkey-eating Eagles “sometimes, but rather rarely, soar over the forest . . . ,” and Grossman
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
TIME OF DAY
Fig. 4. Plot of time of day when eagles were sighted or were heard calling at the Tudaya Falls study area.
and Hamlet (1964) claim that these eagles have “flapping flight with little soaring.” With hut 2 exceptions, I never saw eagles flap their wings, and I fretjuently saw Itirds soaring, though not for extended periods. In traversing an area, they would either glide in a straight line from a higher to a lower elevation or drift over a mountain slope or canyon, riding thermals and mountain updrafts. d4ie most typical glide occurred when the eagles were hunting, and during it the wings were usually fully extended. Sometimes, when greater speed was refiuired, the wings were folded partially or com- l)letely. A gliding or soaring Monkey-eating Eagle holds the wings horizon- tally.
When riding updrafts, the eagles usually circle, hut they occasionally tack hack and forth just above a ridge or glide in a straight line at high altitudes. In these situations, the birds usually either maintain or gain altitude. Circling flight exhibits at least 3 forms. During Part 2 of the hunting cycle, when the bird leaves the forest canopy, it may circle one or more times. During these circles, it maintains its altitude hut drifts horizontally as it turns. It reenters the forest at about the same elevation as the exit point. Circling also occurs during Part 3 of the hunting cycle ( Fig. 3 I when the bird gains altitude. Here the eagle leaves the forest and, finding an appropriate updraft, circles slowly without flapping, gaining height all the time. The diameter of the circles I
Kennedy • BI()IX)(;Y OF MONKEY-EATINC; EA(;LE
7
observed varied from 25 to 40 m. At the highest point, the birds either glide directly toward a mountain slope, usually without loss in altitude and always without flapping, or begin a series of short glides and large sweeping circles. This third form is frecfuently initiated in instances where a bird has gained altitude. Sometimes, a bird drifts from the forest at a higher elevation and glides over the lower slopes, where it begins soaring. The duration of this third type of circling averaged 5 min. Gonzales ( 1968 ) once noted 3 eagles soaring together. I saw only 1 or 2 birds at a time, though 1 received a report ( Engineer Rizardo, pers. comm.) that as many as 7 eagles were seen soaring together.
The Monkey-eating Eagle is relatively short-winged and long-tailed. This structural pattern, which has been termed the Goshawk silhouette by Brown and Amadon ( 1968 ) , is an adaptation of forest-hunting species which reciuire maneuverability and (juick bursts of speed to overtake their prey. Birds with this silhouette usually have flapping flight and seldom soar without flapping. Though the eagle is a forest-hunting bird and is capable of (luick flapping flight in pursuit of prey, it is also a bird that freciuently soars. This soaring ability is, energetically, clearly an adaptive advantage for this species which has a rather large territory.
Vocalizations. — Gonzales (1968), Seth-Smith (1910), and Whitehead (1899), have described the calls of the Monkey-eating Eagle. Whitehead (1899) rendered the call phonetically as “zc-af/ zcafz,” and Gonzales noted it as a long, mellow whistle ending sometimes with a downward inflection hut usually with an upward inflection. He stated that the latter call was the one more frequently given by the breeding pair he was studying. When I heard eagles call, the downward inflection was most frequent. A series of 3 to 9 whistles was repeated at intervals ranging from 45 sec to 5 min. The indi- vidual whistles lasted 0.5 to 1.5 sec and were uttered at 1 to 2 sec intervals. The number of series varied from 1 to 15.
A possible juvenile bird at Tudaya gave a different call. Calling began in the morning with the typical downward inflection; hut as the day progressed, the call changed to a more plaintive whine-whistle, as if the bird was dis- tressed. Each whine-whistle lasted about 2 sec, and a series of these calls was repeated every 45 sec for up to 0.5 h. This type of vocalization resembled that of an eaglet calling after long periods without food as described by Gonzales (1968).
At Tudaya, adult eagles called (1) during Part 1 of the hunting cycle, (2) just after the capture of a prey, and (3 ) during and immediately after being pursued by Rufous Hornbills ( Buceros hydrocorax) . In the first 2 cases, the call had the typical downward inflection. In the third case, the call consisted of a single whine-whistle repeated every 10 sec. This was unlike the call
8
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
Gonzales (1968) described as “short, intense, high-pitched notes” during attacks on eagles by avian enemies.
Interspecific encounters. — The presence of a IMonkey-eating Eagle in an area is often revealed by the noisy mobbing of the bird by groups of up to 20 Rufous Hornbills whose raucous call can be heard as much as 2 km away. The eagle tries to avoid the hornbills by flying from perch to perch within the forest. If unsuccessful, it leaves the canopy and begins circling slowly, gaining altitude as in Part 3 of the hunting cycle. The hornbills follow for awhile, but eventually are outdistanced and retire to the forest. I saw such mobbing on 5 occasions. On the average, the incidents lasted 2 to 3 min, but tbe duration varied from less than 30 sec to just over 15 min. Though I never observed contact, Gonzales ( 1968) reported that Rufous Hornbills actu- ally strike the eagle’s head. He also noted that Writhed-billed Hornbills (Aceros leucocephalus) and Large-billed Crows (Corvus macrorhynchos ) mob eagles. Both these species were numerous at Tudaya, but I saw no inter- actions between them and eagles. I watched an Oriental Hobby ( Falco severus) attack a Monkey-eating Eagle as the larger bird flew slowly at about 60 m above a ridge. Three times the falcon stooped, nearly grazing the eagle’s head. During the harassment the eagle continued flying normally, but it landed shortly afterwards.
HAI5ITAT AM) HOME RANGE
Habitat. — 3 he original habitat of the xMonkey-eating Eagle on Mindanao was undoubtedly dipterocarp forest, which comprised 75% of the virgin forest in the Philippines ( W hitford 1911). Dipterocarp forests are characteristic of moist plains and extend up mountain slopes to 800 m ( Brown and Mathews 1911). Today eagles are mainly confined to the larger mountain masses ( Rabor 1971), but at one time they occupied lowland forest down to sea level. A specimen taken in 1954 in Cotabato City at an elevation of ca. 15 m pro- vided evidence for eagles using lowland forests. The highest elevation at which eagles occur is about 2000 rn, where their preferred prey becomes scarce. At l udaya, I saw eagles enter the forest to begin a hunt at about 1700 m.
As land has been cleared for agriculture and for luml)er, the lower edges of the forests inhabited by the eagles have been retreating up the sides of moun- tains. Tlie birds have partially adapted to this change by hunting over cleared land and living in second growth forest. This adaptive ability was first indicated by Whitehead (1899), who stated, “He [the eagle] is well known to the natives as a robber of their poultry and small pigs . . . ,” thereby implying that the birds forage near clearings. Gonzales ( 1968) described the habitat of the pair he studied as follows: “Some of the hills are still clothed
Kennedy • HIOLOGY OF MONKEY-EATINC; EAGLE
9
with original clipterocarp forest, but others are either naked . . . or covered with coarse cogan grass mixed with shrubs and small trees. The forested hills, however, have not remained virgin for they too have been invaded by the natives as well as logging concessionaires.”
At Tudaya, the eagles’ territory included cleared farmland, various stages of secondary growth, and virgin forest. The birds mainly confined their activity to virgin forest or advanced secondary growth (Fig. 2). Of the 11 eagles I sighted on Mindanao, 10 were in areas of virgin forest or in mixed virgin and advanced secondary forest.
Occasionally eagles were reported in areas where no typical habitat existed. Most of the reports probably resulted from misidentifications, but one con- fiscated eagle ( LSUMZ 73747 ) was shot in a cornfield about 10 km from the closest forest. The abnormal occurrence of the bird at this location is pos- sibly attributable to destruction of habitat in its former territory.
Apparently suitable Monkey-eating Eagle habitat on Mindanao in 1973 (Fig. 5a) comprised 29,000 km- (without allowing for increased area result- ing from elevational differences) or about 30% of the 95,587 km^ of land area of the island. The alarming rate of forest destruction was reported by Gonzales (1971), quoting the Philippine Free Press for 7 June 1969, which stated that the rate of deforestation in the Philippines at that time was 1 ha every 3 min. This problem is not new, however, for Whitehead (1899) stated: “The forests that are left in Samar are still very vast, especially on the Pacific Coast, but for miles inland those of the western coast have been destroyed, leaving ranges of low undulating clay hills chiefly covered with lalang grass. When this country has been passed, the traveler finds himself at an elevation of nearly 1,000 feet and meets with the true virgin forest of Samar. This forest is becoming annually smaller owing to the cultivation of hemp . . . .”
Land clearing has confined suitable habitat on Mindanao to the mountain ranges, but even there the trees have been removed up to at least 500 m in most cases, and sometimes to as high as 1586 m ( Gonzales 1971).
Nine eagles 1 sighted on Mindanao were associated with steep mountain slopes that formed the sides of deep ravines, canyons, or valleys. Data col- lected on the hunting and soaring behavior of the eagle indicate that it is well adapted to such topography, thus I believe that steep mountains are important in the eagle’s habitat.
Home range. — Rabor ( 1968) believes that a pair of Monkey-eating Eagles have a home range comprising from 40 to 50 km^. Gonzales (1968, 1971) says that the range can be as large as 100 km-. Grossman and Hamlet report a smaller range, 31 to 34 km^.
To determine the area used by the pair at Tudaya, I have drawn upon my own observations and those of Parks personnel as well as verbal reports by
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THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
Fig, 5. a. Distribution of tlie remaining habitat for the Monkey-eating Eagle on Min- danao. 1). (Insert) The Philippine Islands, showing the islands where the species has been positively recorded (colored black) and where it was believed present (shaded area).
llie natives living there. Bagolio natives said that the eagles mainly confined their activities to the Fudaya canyon, Parak Creek, and the adjacent ridges and that they were rarely seen elsewhere in the area. Parks personnel first sighted the eagles at Fudaya on 8 April 1972. They found the birds to be restricted to the Tudaya Falls area also. My own observations (Fig. 2) con- firm these reports. I saw eagles on 7 of 8 full days and 10 of 14 half days of surveillance. Clearly, the vicinity around Tudaya Falls has been a center of the eagles’ activity for several years.
I calculated the area of the home range of this pair by 2 methods. Con- necting the outermost points where eagles were sighted (shown in Fig. 2) forms a polygon w ith an area of 6.34 km-, the minimum home range. A sec-
Kennedy • BIOLOGY OF MONKEY-EATINC; EA(;LE
11
ond method was to determine the range hy measuring the greatest distance between sightings. If this length is considered the diameter of a circular home range, the area would be 12.5 km-. Since the range was measured as hori- zontal surface area and does not account for the increased surface resulting from elevational differences, the total area would be greater, perhaps twice as much. The adjusted home range would then be 12.68 km- (polygon method) to 25.0 km- (circular method). These figures suggest that the area necessary to support a pair of eagles may not be as great as formerly believed.
DISTRIBUTION AND STATUS
The Monkey-eating Eagle is known from accounts in the literature to occur on 4 of the larger islands in the Philippines: Luzon, Samar, Leyte, and Min- danao (Fig. 5b). John Hamlet, while visiting the island of Negros in 1945- 1946, received a photograph of a Monkey-eating Eagle allegedly captured on that island. He personally sighted 2 birds soaring over a small island in the Surigao Straits, just north of the province of Surigao del Norte on Mindanao. These reports of eagles from additional islands suggest that the species was probaby more widely distributed in the recent past. An account of the present and former status of the eagle on the islands from which specimens have been taken follows.
Luzon. — Earlier records of Monkey-eating Eagles from Luzon were fre- quently reported in the literature and chronologically follow: 1 killed near
the Agus River in Rizal Province on 11 May 1907, the first specimen posi- tively known from Luzon (McGregor 1907) ; 2 sighted near Montalban, Rizal Province on 13 August 1907 by W. P. Lowe ( Seth-Smith 1910) ; 1 captured on Mt. Ballong, south of Imugan, Nueva Vizcaya Province in January 1917 ( McGregor 1918) ; 1 captured near Pagbilao, Tayabas Province in July 1926 (McGregor 1927); 1 taken from Albay Province (Davidson 1934).
More recent data on the status of the eagle on Luzon were gathered by Rabor (1971) during collecting trips in 1959, 1960, and 1961. In the Cordi- llera and Ilocos mountain ranges in northwest Luzon, he received reports that eagles were last seen in the late 1930’s. However, in the Sierra Madre Moun- tain Range in northeastern Luzon, including the provinces of Cagayan, Isabela, Nueva Vizcaya, and Quezon, the eagles were still being sighted by natives as late as 1960. A specimen captured by personnel from the Philippine Parks in the Isabela-Nueva Vizcaya territory in 1963 is the last known record from this island (Gonzales 1968).
Samar. — The type of the Monkey-eating Eagle was taken on this island by one of Whitehead’s collectors in 1896 ( Ogilvie Grant 1897a). Davidson (1934) cites one other record of the species on Samar; and Rabor (1971), on the basis of the absence of verbal reports in the 20 years prior to his
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
collecting trip in the north central mountains in 1957, considers the bird extinct on Samar. There are no recent data to confirm or refute this sup- position.
Leyte. — For a long time, the only indication that the Monkey-eating Eagle occurred on Leyte was provided by Ogilvie Grant (1897a), who mentioned that J. Whitehead had heard the call of the eagle on this island. Recently, personnel from the Philippine Parks learned of the following records in the province of Southern Leyte: eagles reported sighted in 1951, 1963, and 1968; nest site with young observed in 1969; and 2 eagles killed in 1965 ( S. E. Macanas, Regional Director of the Parks, pers. comm.). Macanas and others sighted the bird in Southern Leyte on 21 November 1970, and he estimated the number remaining there to be 8 to 10.
Mindanao, early population data. — ^There are many reports in the literature (Clemens 1907; Davidson 1934; McGregor 1907, 1921; Mearns 1905; Seth- Smith 1910) of specimens of the Monkey-eating Eagle accjuired throughout Mindanao in the early part of this century, and apparently the first record came from 1 taken near Davao City on 3 September 1904 (Mearns 1905). Published i)opulation reports are lacking for the first 6 decades of its known existence on the island. However, Hamlet, who worked on Mindanao from 1945 to 1946, has informed me that the eagle was not uncommon there, since he located several active nests and knew of many other pairs.
In a recent report, Gonzales ( 1971 ) attempted to estimate the population of Monkey-eating Eagles on Mindanao in 1910, when the forests still covered 65% of the island. Assuming a home range of 100 km‘“ and the use of all the available habitat, he calculated that at least 600 pairs existed on Mindanao in 1910.
Mindanao, recent population data. — According to the 3 surveys mentioned earlier, the population on Mindanao in 1969 and 1970 was between 36 and 60 birds. Careful examination of each report reveals discrepancies among the estimates. In 9 of 17 provinces, the authors agreed on the presence or absence of the eagles; and in 5 of the 9. the estimates were the same. However, Rahor (1971) reported the eagle in 4 provinces where neither of the other investi- gators did. Also, in 4 provinces, 2 of the authors were in agreement as to the eagle’s presence, hut the third considered it to he absent. A striking area of such disagreement is the i)iovince of North Cotahato. There, Alvarez (1970) recorded 11 birds, Rahor (1971) 8, and Gonzales (1971) none.
In Table 1, I have combined the population data of the 3 workers, and without duplicating records collected from specific locations, derived a popu- lation size of 70 birds. This number is 16.6% greater than the highest total and thus clearly shows their estimates to he low. Reasons for their low figures are that they counted only birds that they saw or that were reported to them.
Kennedy • BIOLOGY OF MONKEY-EATING EAGLE 13
I cT
fO
I I CO I
Laiiao del Sur - - 6 6 1800
Misamis 2 2 8 8 400
Occidental
Misamis Oriental - - 2 2 900
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
• S
C:
iin
:£U
III
=fl
z'.
I
Ik
Ik
X
71
I 1
I I
i.
? =
^ X
a I
.11^
VO
On
Si
s ?g
bL !- C = n X
5 «Z1^ n X
f I
S X
II
’§ g
= 3
X X
^1*"''^ numbers are data I collected. Numbers in parentheses are data collected by personnel from the Philippine Research, Parks, Range, and Wi e Division of the Bureau of Forest Development. These unpublished data are subject to revision. Underscored numbers eciual the combined total. Gonzales (1971) considered the population size to he twice the total records he collected; thus 2 X 18 = 36.
Kennedy • lUOLOGY OF M0NKB:Y-EATTNG EAGLE
15
they did not survey all the habitat available to the eagles, and they did not extrapolate their findings to include all the habitat.
Mindanao, present population data. — To determine a reasonably accurate population estimate of the Monkey-eating Eagle on Mindanao during the period January 1970 to April 1973, I used 3 censusing methods. Though each method differed slightly, all used the following formula to derive the total population size:
t = (N/n) t
where T = total population size; t = sample total; N = area of total habitat remaining on Mindanao; and n = area of habitat censused.
Similar to the one Gonzales ( 1971 ) used to estimate population size for 1910, the first method involved determination of the extent of the eagle’s habitat remaining on Mindanao. The amount remaining (Ni) was found to be approximately 29,000 km- (see Fig. 5a and Table 1). Using the maximum home range size of 100 km- (rii ) for one pair (ti = 2), the total population
estimate ( Ti ) equals 580 birds.
A second method involved sampling an area of known size. The Mt. Apo range, west of Davao City proper, was chosen as the site for this study. The amount of suitable habitat in this area was found to be approximately 640 km- (02). Nine eagles were sighted in this area (t2), but as it was physically im- possible to cover the whole mountain range, there were probably more. Since the area of total habitat remains the same, Ni = N2 = 29,000 km-, the total
A
population estimate (T2 ) is 408 birds.
A third method entailed the visitation of as many areas as possible, col- lecting reports of eagles sighted, captured, or killed, and confirming as many reports as possible. The results of this method are shown in Table 1. The combined totals were: number actually sighted by official investigators, 29; number known or reported captured, 16; number known or reported shot, 19; and number of additional eagles reported to the investigators, 74. These data were obtained from 12 of the 17 provinces of Mindanao, hut only 6 provinces ( Davao City, Davao del Norte, Davao Oriental, Davao del Sur, North Cota- bato, and South Cotahato ) were visited regularly, but none was completely surveyed. A rough estimate of the area covered by this method would he 1/3 of the total habitat on the island; thus n^ = 1/3 Ni. Excluding the numbers shot or captured, the total known population in the wild on Mindanao (t;0 was 103 birds ( number actually sighted plus number reported sighted ) during the period of investigation. This gives a 'U of 309 birds.
Implicit in these censusing methods are the following assumptions: ( 1) all the habitat is used by eagles; (2) all the birds in areas sampled were known;
16
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
(3) each pair of Birds occupies a fixed range; (4) results found in one area are applicable to other areas not sampled; (5) population is stable; (6) ranges did not overlap.
The degree of variance among the different methods results from not meet- ing these assumptions entirely, because of insufficient data and the difficulties of censusing. But these techniques do provide a reasonable range (309 to 5o0j in which the number of surviving eagles probably lies.
Mortality should he considered in connection with these estimates since evidence indicates that the population is decreasing. No data are available regarding natural causes of mortality, such as disease and predation, but something is known about 2 unnatural factors. These are: (1 ) loss of habitat (discussed earlier in this i)aper) ; and (2 ) the shooting and capturing of wild birds.
During the course of this study, 3S eagles were known to have been taken from the wild either by being captured or shot (see Table 1). Since the dura- tion of the entire study was 40 months, the average number of birds known to have been lost from the population was ().o7 birds jjer month or 10.4 birds per year. As previously mentioned, these data were collected from about 1/3 of tbe available habitat. Thus, the estimated number removed from the pop- ulation is an average of 31.2 per year. This calculation introduces still an- other assumption, that each eagle, regardless of the degree of its isolation, has an e(iual chance of being captured or shot.
4 he importance of this unnatural mortality depends greatly upon the size and annual recruitment rate of the population. If the i)opulation does lie within the limits suggested by the census methods above, i.e., 309 to 580 birds, then the mortality rate would be from 10.1 to 5.4%.
With high reproductive success and a great percentage of young surviving to adulthood, the 5.1% and possibly tbe 10.1% mortality could be absorbed on an annual basis. However, few data are available concerning the repro- ductive biology of these birds, (irossman and Hamlet ( lOCvl I summarized what was known in 1961. “Although as a rule only one eaglet seems to sur- vive in each nest, theie may be two eggs, and occasionally (in at least one known instance) two young birds. The adults at several nest sites have pro- duced young every year.” Gonzales (1968) found that the pair he studied produced 1 egg and that 1 eaglet per nesting reached fledging age for 2 con- secutive attempts. Kabor ( 1968) also feels that the eagles breed every year. We do not know the age at which these eagles attain sexual maturity nor do we know the proportion of fledglings that survive to that age.
Ihe age of the individuals captured or killed also influences the impor- tance of the unnatural mortality. If all were birds successfully breeding for the first time, the loss would be extremely damaging, as at the 10.1% level it
Kennedy • BIOLOGY OF MONKEY-EATING EAGLE
17
would nearly eliminate this, the most valuable age class. Since the ages of the birds that were captured or shot are not known in all cases, the effect of their loss from the population cannot he determined.
Population ecologists generally agree that a pair of a species reciuires and usually defends a certain semi-fixed area or territory. From this premise, it follows that the number of individuals a system can support is directly pro- portional to the available habitat. On Mindanao, the habitat is being de- stroyed by logging and other land clearing practices to such a degree that many birds are forced to leave their former range and search out new suitable habitat. I believe that a good percentage of the individuals that are captured or shot are birds whose habitat has been destroyed and that have become “surplus.” Thus, presumably, even if they had not been captured or shot, they would not have contributed significantly again to the continuation of the species, unless they were able to establish a new territory in an unoccupied area.
The population surveys conducted have, in the main, shown that the eagles are rather evenly distributed over Mindanao. In some cases, especially parts of Davao del Norte, Davao Oriental, and North Cotahato, the eagles were as common in 1973 in the remaining habitat as they probably had ever been.
CONCLUSIONS AND RECOMMENDATIONS
The total range of the Monkey-eating Eagle has been greatly reduced during the time in which the species has been known. The reduction has been caused by the loss of the eagle’s habitat, and, since the population varies directly with the amount of habitat, it has suffered also. In this paper, I have brought up- to-date most of what is known concerning this species, knowledge that is still extremely patchy. Though the population is larger than formerly believed, the census techni(iues used are based on a modest amount of data, and thus the resulting figure should he considered only a rough estimate. Since the third census included data from about 1/3 of the remaining habitat and was the most extensive survey, I am inclined to regard it as the most accurate. However, no census techni(iue is entirely dependable, especially one based on extrapolation. Taking everything into account, I feel that the population on Mindanao during the period of investigation was about 300 ± 100. The total number of individuals of the species is unknown, as little population work has been conducted on the other islands where it exists or could exist.
Alvarez (1970), Gonzales (1971), and Rahor (1968) have listed the fol- lowing as the principal reasons for the eagles’ decline: (1) the loss of the
eagle’s habitat by logging and agricultural practices; (2) shooting the eagles for trophies; and (3) capturing the eagles for private and public display. In addition, Gonzales (1971) and Rahor (1968, 1971) have presented excellent
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
1
io
recommendations for the conservation of the eagle. Though all their recom- mendations are sound, the 2 that I feel most important are:
1. Educational programs on the conservation of the natural resources in the Philippines, including wildlife.
2. The establishment of Wildlife Sanctuaries and the protection of lands from illegal logging and agricultural practices.
Though shooting and capturing of the eagle certainly contribute to the population decrease, I feel that the primary reason for its decline is the loss of habitat, and therefore, base my recommendations for the conservation of the species on maintenance of its natural environment, as follows:
1. The establishment of preserves in mountain ranges where logging and agricultural practices are not economically feasible. The size of these preserves should he at least 200 km-, in order to encompass enough land for several pairs of birds.
2. For areas used as commercial forests, I suggest that the minimum interval between selective logging be 30 years, to allow regeneration of the native forest.
3. When areas are reforested, I recommend that a diversity of native., Philippine species he planted, thereby recreating as closely as pos- sible, the natural state of the forest.
The destruction of forests in the Philippines is the result of broad social- economic i)rol)lems that need not he described here. However, if the above minimal recommendations are heeded, they should eventually i)ievent any further decline in this rare endemic.
SUMMAKY
A study of the Monkey-eating Eagle was eonducted on the island of Mindanao, Republic of Philipi^ines, from August 1972 to April 1973, in conjunction with the Philippine Re- search, Parks, Range, and Wildlife Division of the Bureau of Forest Development. Infor- mation on the hunting teehnicpies, flight, calls, interspecific encounters, territory size, habitat, and population status are presented.
Though the eagles at one time occui)ied mature forests from sea level to 2000 m, the forests have been destroyed at lower elevations and thus suitable habitat and the eagles are primarily confined to the mountains. The size of the home range of a pair may range from 12 to 100 km^
The eagles are known to have occurred on Luzon, Samar, possibly on Negros, on a small island in the Surigao Straits north of Mindanao, on Leyte, and on Mindanao. Except for L(‘yte and Mindanao, no recent })opulation data are available. An eagle was sighted on Leyte in 1970 and an estimated 8 to 10 birds were thought present at that time on the southern half of that island. On Mindanao, the combined data collected by the author and by jn'isonnel from the Philippine Parks from .lune 1970 to April 1973 suggest that
Kennedy • HIOLOGY OF MONKEY-EATINC; EAGLE
19
earlier population estimates of 36 to 60 for 1969 and 1970 were low, and that the popu- lation size for the period 1970 to 1973 was about 300 ± 100.
The species is declining annually because of destruction of its habitat. Recommenda- tions to prevent further decline are jiresented.
ACKNOWLEDGMENTS
Data for this paper were collected while I was a United States Peace Corps Volunteer assigned to the Philippine Research, Parks, Range, and Wildlife Division of the Bureau of Forest Development. To these agencies I extend my sincere gratitude for allowing me the privilege to study the Monkey-eating Eagle. Special thanks are due Jesus B. Alvarez, Jr. and Armando M. Racelis who supervised the project. Their patience and understand- ing is greatly appreciated. To the participants of the Monkey-eating Eagle Conservation Program, from both the public and private sector, 1 offer my cordial thanks. For his companionship and assistance during most of my work on Mindanao, I am indebted to Antonio 0. Chavez. Robert B. Hamilton, George H. Lowery, Jr., Robert J. Newman, and H. Douglas Pratt kindly read the manuscript and gave many helpful suggestions. Hugh M. Turner helj)ed with photographic processing. Finally, I wish to thank Linda Anne Kennedy for her help during all phases of the study.
LITERATURE CITED
Alvarez, J. B., Jr. 1970. A report on the 1969 status of the Monkey-eating Eagle of the Philippines. Pap. and Proc. Int. Union Conserv. Nat. Nat. Resour. 11th Technical Meeting, New Delhi, India, 25-28 November 1969, Vol. 11:68-73.
Brown, L. and 1). Amadon. 1968. Eagles, hawks, and falcons of the world. McGraw- Hill Book Co., New York.
Brown, W. H. and I). M. Mathews. 1914. Philippine dipterocarp forests. Philipp. J. Sci. 9:413-568.
Clemens, J. 1907. Notes from the Pliilij)j)ines. Condor 9:92-93.
Davidson, M. E. M. 1934. Specimens of the Philippine Monkey-eating Eagle {Pithe- cophaga jefferyi) . Auk 51:338-342.
Gonzales, R. B. 1968. A study of the breeding biology and ecology of the Monkey- eating Eagle. Silliman J. 15:461-491.
. 1971. Report on the 1969 status of the Monkey-eating Eagle on Mindanao Is- land, Philippines. Bull. Int. Counc. Bird Preserv. 11:154-168.
Grossman, M. L. and J. Hamlet. 1964. Birds of prey of the world. New York, Bonanza Books.
McGregor, R. C. 1907. Notes on s])ecimens of the Monkey-eating Eagle ( Pithecophaga jefferyi Grant) from Mindanao and Luzon. Philipp. J. Sci. 2:297.
. 1918. New or noteworthy Philippine birds, II. Philipp. J. Sci. 13:1-20.
. 1921. New or noteworthy Philippine birds, IV. Philipp. J. Sci. 19:691-703.
1927. New or noteworthy Philippine birds, V. Philipp. J. Sci. 32:513-528.
Mearns, E. a. 1905. Note on a specitmm of Pithecophaga jefferyi Ogilvie-Grant. Proc. Biol. Soc. Wash. 18:76-77.
Ogilvie Grant, W. R. 1897a. On the birds of the Philippine Islands.— Part IX. The islands of Samar and Leite. Ibis 3 (7th Series) : 209 250.
. 1897h. I New specimens of birds from the island of Samar.l Bull. Br. Or-
nithol. Club No. XL. Ibis 3 (7th Series) :253-254.
Raror, I). S. 1968. The present status of the Monkey-eating Eagle, 1*ithecophaga jef-
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
feryi Ogilvie-Grant, of the Philippines. Int. Union Conserv. Nat. Nat. Resour. Publ. New Ser. No. 10:312-314.
. 1971. The present status of conservation of the Monkey-eating Eagle of the
Philippines. Philipp. Geogr. J. 15:90-103.
Seth-Smith, D. 1910. On the Monkey-eating Eagle of the Philippines ( Pithecophaga jef feryi) . Ibis 4 (9th Series) :285-290.
Whitehead, J. 1899. Field notes on birds collected in the Philippine Islands in 1893-6. Ibis 5 (7th Series) :81-111.
Whitford, H. N. 1911. The forest of the Philippines. Part 1. Forest types and prod- ucts. Uept. of the Int., Bur. of Forestry Bull. No. 10. Bureau of Printing, Manila.
MUSEUM OF ZOOLOGY, LOUISIANA STATE UNTV., BATON ROUGE 70893. ACCEPTED 28 SEPT. 1976.
NEW LIFE MEMBER
Robert S. Kennedy is a new life member of the Wilson Ornithological Society. He is presently completing his doctoral work at Louisiana State University. His research int('iests include studies of several species of raptors, including the Monkey-eating Eagle of the Philippines. Other studies deal with avian population dynamics. Mr. Kennedy has had extensive field experience in the Philippines and in North and South America. In addition to his research interests, Mr. Kennedy enjoys traveling, and collecting and marketing wildlife art.
COWBIRD PARASITISM AND EGG RECOGNITION OF THE NORTHERN ORIOLE
Stephen I. Rothstein
Little information exists on host-parasite interactions between the Northern Oriole {Icterus galbula) and the parasitic Brown-headed Cowhird {Molothrus ater) (Friedmann 1963) even though both species are abundant and broadly sympatric. The small number of nests known to have been parasitized is not due to a scarcity of observations on oriole nests; e.g., parasitism was observed at only 8 (2.5%) of 318 oriole nests in Ontario (Peck 1974). Alternative ex- planations can account for the scarcity of observed parasitism: ( 1) Northern Orioles typically accept cowhird eggs but are rarely parasitized; or (2) North- ern Orioles typically eject cowhird eggs causing a large proportion of cow- bird eggs to disappear before observers see them. Under the first explanation, the frequency of observed parasitism would equal the frequency of actual parasitism. But under the second, incidences of observed parasitism would always be less than incidences of actual parasitism and orioles might be fre- quently parasitized even though parasitism is rarely seen.
If orioles typically eject cowhird eggs, the cases of natural parasitism most likely to be seen would he those rare ones in which parasitic eggs are accepted. Thus observations of natural parasitism do not give reliable data on the fre- quency with which birds eject cowhird eggs. Reliable data on ejection can be derived by experimentally placing cowhird eggs into nests because the experi- menter can determine the fate of all the cowhird eggs within a sample. In 1968 I placed an artificial cowhird egg in a Northern Oriole nest. The egg was ejected within 24 h. This was one of a series of experiments on many species. These experiments demonstrated little intraspecific variation in re- sponse to experimental cowhird parasitism (Rothstein 1975a, 1975b). Thus species were easily divided into 2 discrete groups — accepters and rejecters. Based on the 1 experiment and on several reports of ejections of naturally deposited cowhird eggs (Friedmann 1963, Smith 1972), I tentatively desig- nated the Northern Oriole as a rejecter species. New experiments on 27 addi- tional nests reported here demonstrate that this designation was correct. Ex- periments on 2 nests also deal with behavioral mechanisms responsible for the oriole’s egg recognition.
MATERIALS AND METHODS
Artificial eggs. — Artificial cowhird eggs (Fig. 1) made of plaster of Paris were used in most experiments. These eggs are identical to ones described elsewhere (Rothstein 1975a, 1975c) except that eggs used in nests whose number begins with “74-” or “75-”
21
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THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
Fig. I. Egg Types Mentioned in Text. Top row: 2 cowbird eggs; an artificial cow- bird egg; 2 artificial cowl)ird eggs damaged by orioles (The surface of the artificial egg on tlu‘. left lias been blackened so as to better reveal the peek marks. The most heavily damaged ai(*a of the egg on the right is within the black lines.). Bottom row: House
Sparrow egg after being ejected from nest 75-81; 2 Northern Oriole eggs; Red-winged Blackbird egg; Loggerhead Shrike egg from same clutch as eggs placed in nest 74-86.
were coated with a clear aerylie polymer gloss medium, not with shellac. Controls per- formed on other sjiecies show birds do not reject these eggs because they are artificial (Rothstein 1975a, 1975c). 1 performed controls on orioles by experimentally parasitizing nests with a real cowbird egg or with real House Sparrow (Passer domesticus) eggs. The latter simulate cowbird eggs in color and size (Fig. 1, see data in Bent 1958).
Experimental procedures. — During a single visit to each nest, one “parasite” egg was added and one “host” (oriole) egg was removed. I experimentally parasitized most nests between 12:00 and 18:30. Elsewhere (Rothstein 1975a) 1 discussed differences between my procedures and those usually employed by cowhirds; hut these differences have no detectable influence on the incidence of rejection. Experimentally parasitized nests were usually checked wdthin 24 ±; 2 h. If an experimental egg was not ejected I left it in the nest for at least 7 days except at nest 73-01 where nest checks ceased after 3 days.
Nest stage. — Most naturally parasitized nests receive cowbird eggs during the host’s laying period (Friedmann 1963). The question of whether there is a correlation be- tween host response and nest stage can he answered by parasitizing nests throughout the cycle. I divided nests into 3 stages. Those known to he parasitized on or before the day
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cowbird egg; E = egg ejected; D = egg damaged and left in nest.
» Additional eggs could have been laid in nests where precedes clutch size. Uncertainties exist for various reasons, e.g. visits to one nest ceased before it was certain that egg laying was completed (68-220). At other nests parasitized during the laying stage, egg disappearance may have been impossible to detect because eggs lost could have been replaced by eggs laid between nest checks.
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
the last oriole egg was laid were in the laying stage, “L” in Table 1. Nests at which no additional oriole eggs appeared after parasitization were in the incubation stage, “I,” if the oriole egg I removed contained an embryo. The third stage was intermediate, “L-I,” and consisted of nests at which no additional eggs appeared but at which the removed oriole egg was fresh and lacked an embryo. 1 estimate nests in the L4 stage were para- sitized between the day the last egg was laid and 2 or 3 days later.
Study areas. — Most nests were studied during May 1974 and May-June 1975 within 18 km of Shandon, San Luis Obispo Co., California. Nests studied in other areas are as follows: 68-220, Woodbridge, New Haven Co., Connecticut; 72-01, Delta, Manitoba; 73- 33, Goleta, Santa Barbara Co., California; 73-01, Chaffey’s Locks, Ontario, and 75-103, Tupman, Kern Co., California.
RESULTS OF EXPERIMENTS SIMULATING NATURAL COWRIRD PAIUSITISM
Responses to experimental parasitism. — Rejection occurred at all 5 nests where I added a real cowbird or cowhird-like egg (Table 1). In addition, Raleigh J. Robertson and Richard Norman (pers. comm.) added real cow- hird eggs to 5 oriole nests at Delta, Manitoba. Each was rejected. As Robert- i
son and Norman’s tecbni(iues differed slightly from mine, I do not include 1
their data in statistical tests that follow. Because rejection of real eggs oc- (
curred at 10 of 10 nests, I assume rejections of artificial cowbird eggs at IB l
other nests TITihle 1) were not in response to artificiality of the eggs. Thus, '
orioles rejected at each of 2B nests sampled. The 95% confidence interval for ‘
percent rejection in the total population is B6 to 100% (determined from Owen 1962 ). All the real eggs in my experiments were ejected whereas only 4 of IB artificial eggs were ejected; the remainder were damaged and left in the nest. The incidence of ejection differs significantly for the 2 egg types {P < 0.02; Fisher exact test, Hailey 1959 and tables in Owen 1962. All suh- se(iuent probabilities also involve this test.). I searched for ejected eggs in an area 3 to 5 m around the point below each nest hut failed to find them except in 1 case in which I watched the oriole eject (next section). Most of the damaged artificial eggs had numerous shallow peck marks (Fig. 1). To determine the total number of pecks I counted the peck marks in an area centered around the e(iuator of the egg from nest 74-Bl. This egg had under- gone moderate to heavy damage. I extraiiolated this figure to the entire egg (using a formula in Romanoff and Romanoff 1949 for surface area). During the 7 days it was in the nest at least 196 pecks were inflicted, or half this number if each peck was w ith the hill open. Besides these shallow peck marks, most of which did not iienetrate the paint to the underlying plaster, the eggs from 3 nests also had gouges up to 1 mm into the plaster indicating these birds concentrated pecks at specific sites.
Observations of rejection behavior. — After parasitizing a nest I usually left the area as quickly as possible, not returning until a subsequent day. I did
i
Rothstein • COWIUKI) PARASITISM OF ORIOLES
25
watch 6 nests immediately after 1 inserted the experimental egg and at 2 other nests I returned within an hour: (1) At lo:3() I parasitized nest 75-81 (Ta- ble 1) with a House Sparrow egg while a female scolded. About 4 min after I left the nest the female landed on the nest rim, looked into the nest for several sec and then “up-ended,” clinging to the inside of the nest wall with her body roughly perpendicular to the ground and her tail protruding from the nest. The motion of her tail indicated she began to peck immediately at the eggs. Pecking continued for about 75 sec; then she flew from the nest and fluttered within several m of it for about 30 sec. She then returned to the nest, immediately up-ended and began to peck. After about 40 sec she left the nest with the House Sparrow egg in her hill and flew to a branch about 10 m away. Upon landing she seemed to immediately wipe the egg across a twig and then dropped it. 1 retrieved the sparrow egg from the ground and found part of the shell at the pointed end missing (Fig. 1). The missing shell may have remained in the tree. At 18:57 I returned to the nest, and added an artificial cowbird egg. Until 19:04 the female fluttered within 5 m of the nest, frequently looking into it. She then flew to the nest, immediately up- ended as before and began to peck. She stayed in the nest about 2 min, fre- quently pecking in rapid series of 2-4 pecks. She then left the nest and fluttered nearby only to return at 19:10 again up-ending immediately and pecking until she left at 19:12. She had not returned to the nest when I ceased observations at 19:15. Pecks against the artificial cowbird egg were delivered with such force that they were audible about 6 m away. During these observations I did not see or hear a second oriole.
(2) At 10:53 a real House Sparrow egg was inserted into nest 75-84, while the female scolded. For the next 34 min the female fluttered within 3 to 10 m of the nest, vocalizing frequently. Several times she perched about 1 m over the nest, tilted her head and apparently inspected the nest contents. She was evidently reluctant to return to the nest, perhaps frightened by a rope we had tied to a nearby branch and by occasional disturbances from a nearby house. We removed the rope between 11:27 and 11:31. At 11:42 the female landed on the nest rim, stood there and began to frequently bend over and peck into the nest. She did not up-end as did the bird at 75-81, probably be- cause this nest was not as deep. At 1 1 :44 she flew from the nest, landed about 12 m away and began to hill wipe for several min. At 11:48 and 11:50 she again stood on the nest rim, pecked into the nest and then flew^ suddenly. In none of her 3 departures were we able to determine whether she carried an egg. At 11:55 we inspected the nest. The House Sparrow egg and one oriole egg were missing. Pieces of oriole eggshell were found beneath the branches the female flew to after her second and third pecking sessions. Perhaps the sparrow egg was removed after the first pecking session and the oriole egg
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THE WILSON BULLETIN • VuL 89, No. I, March 1977
was l)roken while the bird tried to eject the sparrow egg. A second oriole was not detected.
(3) I added a real cow Bird egg to 75-109 at 15:45 while 1 or 2 orioles scolded. At 15:47 a female landed on the nest, up-ended and began to peck. She flew after about 90 sec hut I couldn’t see if she carried an egg. This per- formance was repeated about 1 min later and again 1 couldn’t detect whether an egg was carried away. At 15:50 the female went to the nest and began to incubate. At 16:02 I ceased my observations, chasing the female from the nest which now^ contained only 4 oriole eggs. A striking feature at nests 75- ol, 84, and 109 was the speed with which the females left the nest after most pecking sessions. I suspect they carried eggs or parts of eggs on these de- partures and left (juickly so as to avoid dropping eggs back into the nest.
(4) Nest 75-112 was parasitized at 11:00 with an artificial egg. An adult male scolded while I was at the nest. I watched the nest until 11:14. During this time the male stayed in the tree with the nest hut never came within 2 m of it. As the male was not scolding and showed no “nervous” behavior, my presence about 25 m away was probably not responsible for his failure to go to the nest. A female was not detected.
(5) At 15:24 I added an artificial egg to nest 75-123 while scolded by a female. I did not see the female arrive at the nest hut 3.5 min later I noticed her, up-ended and pecking into the nest. After about 30 sec she flew^ to a tree roughly 60 m away and was joined by an adult male. About 30 sec later she returned to the nest, up-ended for about 90 sec and then went all the way into the nest and apparently sat on the eggs. I flushed the female from the nest at 15:34.5. ITie cow bird egg had 15-30 peck marks. The one oriole egg I had left in the nest was undamaged.
(6) I i)arasitized nest 75-101 at 18:08, flushing a female from the nest, rhe nest was watched until 18:20 hut no orioles were seen.
(7) Nest 75-88 was parasitized with a House Sparrow^ egg at 18:15. A male and female scolded while the egg was inserted. The sparrow egg was missing when 1 returned at 18:49 and a female was incubating.
(8l I parasitized nest 75-103 at 10:25 with a House Sparrow' egg. No orioles were detected. When I returned at 10:35 a bird was on the nest and the undamaged sparrow egg was present. At 11:00 the egg was gone and a bird w as again on the nest.
Idiese observations suggest ejection usually occurs shortly after a female returns to her nest as w as the case at nests 75-81, 84, 109, and 123. The speed with which birds carried ejected eggs made it impossible to determine how the eggs were carried. Pecking motions that preceded ejections indicate eggshells were pierced before removal but whether the eggs, still virtually intact, were speared on the hill or whether the eggs were broken in the nest
Rothstein • COWBIKI) PAMASITISM OF ORIOLES
27
and pieces carried away separately is uncertain, d'he former is more likely but the latter may have occurred at 75-109 as the oriole made 2 rapid de- partures from the nest. My observations indicate most ejected eggs are dropped at least several m from the nest. Orioles are known to tlrop natu- rally deposited cowhird eggs directly from the nest (Friedmann 1963, Smith 1972) hut these cases of natural parasitism were detected only because eggs were dropped from the nest. Each of the 4 rejections I observed was by a female, suggesting males do not usually reject. Furthermore a male, hut not a female, was present and scolded when I parasitized nest 75-112, yet the male did not inspect the nest as had females at other nests. Whether males totally lack rejection behavior remains an important (luestion and is critical to the population genetics of the rejection trait (Rothstein 1975b).
Breakage and disappearance of oriole eggs. — Some but not all oriole eggs disappeared from or were l)roken in 11 of 18 nests parasitized with artificial eggs. I suggest orioles broke their own eggs while attempting to eject artificial eggs and that they later removed some of these broken eggs. Birds remove their own eggs if these have holes ( Poulsen 1953, McClure 1945). This inter- pretation is supported by several lines of evidence. The incidence of missing or broken oriole eggs at nests parasitized with real eggs (1 in 5) is signifi- cantly (P < 0.05) less than for nests that received artificial eggs. That orioles removed their own broken eggs is suggested by the fact that at some nests, eggs seen to be damaged on one nest check were missing on a subse- quent check. Finally the female at nest 75-81 ejected a House Sparrow egg without breaking any of her own eggs. I then added an artificial egg. The next day the nest contained a damaged artificial egg and pieces of oriole eggshell. Pieces of oriole eggshell were also on the ground beneath the nest, which was deserted. Breakage of oriole eggs probably occurred when an oriole’s bill or the plaster egg rebounded against the oriole eggs during peck- ing or when a plaster egg was dropped on the oriole eggs. Possibly orioles actively pecked their own eggs during redirected behavior occurring when their frustrated attempts to eject the plaster egg conflicted with another tendency such as incubation.
Effects of nest stage. — As orioles parasitized during all 3 nest stages re- jected (Table 1) there is no correlation between nest stage and acceptance or rejection of cowbird eggs. However, there is a possible correlation between nest stage and amount of effort exerted in rejection. The fact that artificial eggs can’t be ejected easily provides a measure of rejection effort because different amounts of effort may produce different results. By contrast, with real cowbird eggs, rejection effort, even if it does change with the breeding cycle, may always be sufficiently strong to result in the same response — rapid ejection. In response to artificial cowbird eggs, intense rejection effort is
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THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
likely to lead to ejection, rather than only egg damage. The incidence of missing oriole eggs may be correlated with rejection effort because the more intense the attempts to eject the plaster egg the more likely oriole eggs will be damaged. Results from the 13 nests that received artificial eggs and were checked on day 1 (24 h after parasitization ) are relevant. On day 1, 5 of 7 L and L-I nests showed ejection of the plaster egg or disappearance of at least 1 oriole egg whereas these events occurred at none of 6 I nests \P < 0.025). This suggests rejection effort decreases after the first 3 days of incubation.
THE ISOKTIIEKN OUIOLE AS A REJECTER SPECIES
Because rejection occurred at each experimental nest the Northern Oriole conforms to the resiionses of previously designated rejecter species. These species reject cowhird eggs at rates of 88 to 100% ( Rothstein 1975h ) . Ex- periments on orioles were conducted in 4 widely spaced regions (California, Ontario, Manitoba, Connecticut) suggesting rejection is characteristic of the entire species. However, because only 1 nest was tested in 2 regions and because the species is polytypic in morphology ( Rising 1970, Misra and Short 1974) experiments in other regions should he done.
There is little doubt that orioles that damaged artificial eggs would have ejected real ones. Iliis must mean that the natural parasitism that is observed is just a fraction of the actual parasitism that occurs. The rapidity with which cowhird eggs can he ejected is shown by nests observed immediately after they were parasitized experimentally, I he point is also demonstrated by the fact that rejection occurred within 24 h at 17 of 18 experimental nests visited on day 1 CFahle 1). Methods to estimate the rate of natural parasitism are described elsewhere (Friedmann et al. 1977).
I he Northern Oriole's status as a rejecter contrasts with other Icteridae. Two well studied icterids. Red-winged Blackbird {Afielaius phoeniceus) and Common (irackle \(JuiscaIus (/uiscula) are accepter species (Rothstein 1975a). The contrast between the oriole and Red-wing is especially interesting because their eggs are similar (Fig. 1). Ihe presence of a definite rejecter species within the Icteridae strengthens the generalization (Rothstein 1975a) that species within a family often differ as regards rejecter-accepter status.
COMPARISONS BETWEEN THE NORTHERN ORIOLE AND OTHER REJECTER SPECIES
Fourteen of 18 (77.8%) oriole rejections of artificial cowhird eggs were by damage. Only 6 of 201 (3.0%) rejections of artificial cowhird eggs by 7 other rejecters were by damage and all of these were by the Cedar Waxwing { Bombycilla cedrorum) (data in Rothstein 1975a ). Orioles rejected by dam- aging significantly {F < 0.005) more frequently than every other rejecter
Rothstein • COWBIRD PARASITISM OF ORIOLES
29
species, except the Western Kingbird (Tyrannus verticalis ) , for which I tested only 2 nests. The waxwing and oriole differ in the type of damage they inflicted. In contrast to the numerous shallow peck marks on cowhird eggs damaged by orioles, eggs from waxwing nests had nearly all the damage re- stricted to several large depressions dug into the plaster. Damaged eggs were probably more prevalent among orioles because this species ejects cowhird eggs by spiking them. Other rejecters usually lift cowhird eggs in their mandibles ( Rothstein 1975a ) . While the occurrence of damaged cowhird eggs left in nests is probably an artifact of using plaster eggs ( i.e., real cow- bird eggs would have been removed ) it leads to the discovery that the oriole differs from other rejecters in its ejection technique — a finding that would not have resulted as easily from experiments using real cowhird eggs.
Ejection by spiking would not seem to be as adaptive as ejection by carry- ing eggs in the mandibles. Even if a broken egg is quickly removed it may leak its contents and this endangers the other eggs ( Rothstein 1975a ) . A bird spiking an egg might cause the egg or its bill to rebound against other eggs, thereby breaking them. Why then does the Northern Oriole eject by spiking instead of by carrying eggs in its bill? I suggest a bird would have diffi- culty removing an egg from the deep pendant nest characteristic of orioles unless the egg were securely impaled on the bird’s hill. Otherwise, the egg might fall back into the nest and damage the bird’s own eggs. By contrast other rejecter species I studied have the cup-shaped, shallow nests typical of most passerines. Corroborative evidence is provided by N. G. Smith’s findings (pers. comm.; see also Smith 1968) that oropendolas and caciques, whose nests are even deeper than those of the oriole, also eject by spiking. The shape of the oriole’s bill may also introduce some difficulties in ejection. Other re- jecters have either slightly decurved or hooked bills but the oriole’s bill is straight and this may make it difficult for orioles to lift eggs. Also, among known rejecters the oriole has the smallest bill after the Cedar Waxwing.
The oriole and waxwing differ from other rejecters in the incidence with which some but not all of their own eggs were found broken or missing from the nest. Missing or damaged “host” eggs occurred at 12 of 23 ( 52.2% ) oriole nests and at 25 of 58 ( 43.2% j waxwing nests subjected to experimental cowbird parasitism. Breakage or disappearance of host eggs occurred at only 5 of 190 (2.6%j experimental nests of the other rejecters (Rothstein 1976).
The loss of oriole eggs in experimentally parasitized nests is not totally lacking in biological significance. One nest parasitized w ith a real egg ( 75- 84, Table 1 ) lost an oriole egg during the ejection process. The remaining oriole eggs had wet egg contents on them and this may have caused further losses. Another nest 1 parasitized w ith a real egg ( 75-109) showed a poten- tial for the loss of oriole eggs. About 15 min after the female ejected a real
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
covvbircl egg, 2 of the 4 oriole eggs had wet egg white on them. A third had a small feather and some cottony nest lining glued to it. These eggs were incu- hated successfully but my handling may have decreased the likelihood that they would be glued to one another or the nest. I suggest that orioles reject cowbird eggs at some risk to their own eggs. This risk explains the possible reduction in the oriole’s rejection effort during the I stage (see above). Selec- tion may favor a reduction in rejection effort during the I stage because cow- bird eggs laid then pose little threat to the oriole’s reproductive output but sustained efforts to eject such eggs could result in loss of oriole eggs. A simi- lar explanation accounts for the fact that Cedar Waxwings shift from 87.5% rejection during the L and L-I stages to 40.0% rejection during the I stage ( Kolhstein 1976).
TIJUE FXG UECOG.MTION VERSUS RECOGNITION ON THE BASIS OF UISCORDANCY
Oriole and “parasitic” eggs in experiments reported above differed in ap- pearance and numerical representation in tbe clutch (the parasitic eggs were outnumliered by oriole eggs). Lbus do orioles reject eggs on the basis of appearance or on tbe basis of wbich egg is in the minority? I shall refer to these 2 mechanisms as true egg recognition and recognition on the basis of discordancy. If the latter occurs orioles should reject their own egg if it is outnumbered by foreign eggs. Experiments on other species demonstrated true egg recognition (Victoria 1972, Kothstein 1975c).
Data for 2 nests (75-107 and 123 I in Table 1 indicate true egg recognition. Artificial cowbird eggs were rejected even though only 1 oriole egg was l)iesent. l bus the orioles at these nests rejected the foreign egg even though it and their own egg type were represented ecjually.
After artificial cowbird eggs had been rejected, exi)eriments were con- ducted at 2 nests to distinguish between the 2 modes of egg recognition. On 13 May nest 71-79 (Table 1) contained 5 oriole eggs. At 16:55 I replaced 4 of these with 3 Loggerhead Shrike \ Iauuus ludovicianus) eggs. When next checked, on 11 May at 11:11. the nest contained only an undamaged oriole egg. Ihe nest was still active as 2 orioles scolded intensely. I found no trace of the missing shrike eggs in an area 3 to 5 m around the point under the nest. When next visited on 20 .May the nest was abandoned and curiously the still intact oriole egg was buried under 10 to 25 mm of new nesting material. On 13 May. nest 71-86 contained 1 oriole eggs and one heavily damaged cowbird egg ( Table 1). I removed the latter at 12:16 and at 12:50 1 replaced 3 oriole eggs with 1 shrike eggs. At 13:27 I removed 1 shrike egg l)ecause the com- bined mass of 4 shrike and 1 oriole egg was too large for the eggs to lie on the nest floor in 1 layer. Ihe eggs were being inculcated when checked at
Rothstein • COWBIRl) I’ARASniSM OF ORIOLES
31
13:27. When next visited on 14 May at 18:35 the nest contained only the oriole egg. The egg was cold and had a hole that measured about 3 hy 2 mm. No orioles were in attendance. The shrike eggs were not found beneath the oriole nest.
The orioles at nests 74-79 and 74-86 demonstrated true egg recognition. Both nests were probably eventually abandoned because the single oriole egg that remained was not a sufficient stimulus to release incubation behavior. The shrike eggs were larger than the orioles’ eggs. At nest 74-79 the oriole egg left with the shrike eggs measured 22.67 X 15.95 mm. Measurements are unavailable for the 3 shrike eggs placed in the nest but 2 eggs from the same shrike clutch measured 23.70 X 18.40 and 24.92 X 18.97 mm. Measurements are unavailable for the shrike eggs used at nest 74-86 but these eggs were also larger than the oriole egg (unpubl. photograph). Thus these experiments present no evidence that orioles prefer large eggs or that large eggs are a supernormal stimulus, as has been found in some nonpasserines (see Tin- bergen 1951).
SUMMARY
Experiments on 28 Northern Oriole nests sliowed this species does not tolerate cowhird parasitism. Artificial or real cowhird eggs or real House Sparrow eggs, which simulate cowhird eggs, were rejected at every nest. Real eggs were ejected whereas most artificial (plaster) ones were damaged and left in the nest. Ohservations at nests immediately after they were parasitized showed: (1) 4 of 4 rejections were hy females, (2) cowhird eggs are often ejected within min, (3) cowhird eggs are usually dropped at least several m from the nest. The oriole’s rapid removal of cowhird eggs indicates that the natural parasitism that is observed is a fraction of the total parasitism that occurs.
The Northern Oriole corresponds closely to species previously designated as rejecters — these species reject cowhird eggs at rates close to 100%. But other rejecters usually remove artificial cowhird eggs whereas most orioles damaged them and left them in the nest. This difference demonstrates orioles eject cowhird eggs hy spiking although other species do so by lifting the egg in their mandibles. The oriole’s special ejection techniciue is probably an adaption to its pendant nest. Although orioles reject cowhird eggs throughout the egg stage, the effort exerted in rejection seems to weaken during incuba- tion. This decrease in rejection effort may have been selected for because cowhird eggs laid during the oriole’s incubation pose little threat to the oriole’s offspring hut ejecting them endangers the oriole’s own eggs. Orioles correctly distinguished between their own and foreign eggs even when the latter outnumbered their eggs, as orioles at 2 experi- mental nests ejected 3 and 4 real Loggerhead Shrike eggs even though only 1 oriole egg was present.
ACKNOWLEDGMENTS
Much of the data could not have been gathered without the able and indispensable field assistance provided by Donald A. Schroeder. The generosity and hospitality of Clare Hardham, Ian and Donald McMillan, and many other ranchers made it possible for me to conduct my studies near Shandon. These individuals also provided valuable infor- mation on local ecological conditions. Richard S. Miller kindly conducted the experiment
32
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
on nest 72-01 and gathered data on additional oriole nests. Sharron Elliott and Raleigh J. Robertson conducted the experiment on nest 73-01. The manuscript profited from critical reviews hy Lloyd F. Kiff, Dennis ]M. Power, Donald A. Schroeder, and Robert A. Wallace. 1 am thankful to all these individuals who have helped me at various stages of this project. Financial aid was provided hy a Faculty Research Grant from the University of California.
LITERATURE CITED
BAiLt:Y, N. T. J. 1959. Statistical methods in biology. English Univ. Press, London. Bknt, a. C. 1958. Life histories of North American blackbirds, orioles, tanagers, and allies. U. S. Natl. Mus. Bull. 211.
Friedmann, H. 1963. Host relations of the parasitic cowhirds. U. S. Natl. Mus. Bull. 233.
•, L. Kiff, and S. 1. Rotiistein. 1977. A further contribution to knowledge
of the liost relations of the jiarasitic cowhirds. Smithson. Contrih. Zook, No. 235. McClure, H. E. 1945. Reactions of the Mourning Dove to colored eggs. Auk 62:270- 272.
Mlsra, K. K. and L. L. Short. 1974. A hicunetric analysis of oriole hybridization, (iondor 76:137-146.
Owen, I). B. 1962. Handbook of statistical tables. Addison-Wesley Puhl. Co., Reading, Mass.
Peck, C. K. 1974. Ontario nest records scheme, eleventh report (1956-1974). Royal Ontario Museum. Toronto.
PotJi.sEN, 11. 1953. A study of the incubation responses and some other behaviour pat-
terns in birds. Vidensk M(‘dd. Dan. Naturhist. Foren. KBH 15:1-131.
Kisinc, j. I). 1970. Morphological variation and evolution in some North American
orioles. Syst. Zook 19:315-351.
Komanoff, a. L. and a. j. Komanoff. 1949. The avian egg. John Wiley and Sons,
N. Y.
|{(»Tiis'iEiN. S. I. 1975a. An experimental and teleonomic investigation of avian brood parasitism. C(»ndor 77:250-271.
. 19751). Evolutionary rates and host defenses against avian brood parasitism.
Am. Nat. 109:161-176.
. 1975c. .Mi'chanisms of avian egg-recognition: do birds know their own eggs?
Anim. Bchav. 23:268 278.
. 1976. Expcriimuits on host defenses CaMlar Waxwings use against cowhird
parasitism. Auk 93:675 691.
Smith, N. (i. 1968. 'khe advantage of being parasitized. Nature 219:690-694.
Smith, T. S. 1972. Cowhird parasitism of Western Kingbird and Baltimore Oriole nests. Wilson Bulk 84:497.
Tinrercen, N. 1951. The study of instinct. Oxford Univ. Press, London.
Victoria, J. K. 1972. Clutch characteristics and egg discriminative ability of the African Village Wi'averhirtl P/oceus ciicullatiis. Ibis 114:367-376.
DEDT. OF mOLOGICAL SCIENCES. UMV. OF CALIFORNIA, SANTA BARRARA 93106. ACCEFTEl) 20 JAN. 1976.
OBSERVATIONS ON THE RED-NECKED GREBE NESTING IN MICHIGAN
Michael L. Chamberlin
The Red-necked Grebe [Podiceps grisegena) in Michigan is a regular transient although generally uncommon. Zimmerman and Van Tyne (1959j give only 5 summer sight records through August 1958. From 1959-1974 Michigan Summer Bird Surveys recorded only one observation, a group of 12 on 30 August 1962 that were likely migrants ( Mahan 1963 j . The nearest nesting records are for Wisconsin, Minnesota, and Ontario (Jones 1938, Speirs et al. 1944, A.O.U. 1957). The following account is the first record of Red-necked Grebes nesting in Michigan.
STUDY AREA AND METHODS
On 16 June 1975, Steve Goodman and 1 located a Red-necked Grebe nest containing 7 eggs in a marshy section of Cedarville Bay, Cedarville, Mackinac Co., Michigan. Four days later, on 20 June, we sighted 2 adult Red-necked Grebes in the same marsh. The marsh covered approximately 15 ha of the west shore of the hay (Fig. 1). The near-shore area of the marsh was a dense growth of cattail {Typha latijoUa) and sedge {Carex sp.). The deeper waters contained pondweed i Potamogeton sp.), bulrush iScirpus sp.), pickerelweed {Pontederia sp.), smartweed (Polygonum sp.), spatterdock (Nuphar sp.j, water milfoil ( Myriophyllum sp.), and hladderwort ( Utricularia sp.).
I observed the pair almost daily from 20 June to 29 August 1975, for a total of 259 hours and 37 min. A single sighting was also made on 28 September. I attempted to distribute observations evenly throughout the day from 06:00 to 22:00. Observations before and after the incubation period, when the birds were the most mobile, were made with 7 X 35 binoculars from a canoe. Observations during incubation were made with a 20 X scope from a black rowboat anchored among the cattails 67 m from the nest. The birds appeared to become accustomed to the boat and frequently swam within several meters of it. To avoid losing this familiarity the more disturbing visits to check nest contents were made from the aluminum canoe and the nest was approached from the opposite direction of the observation boat’s route. Daily nest checks were made until the first egg was laid, after which the nest contents were checked once a week. Although the Red-necked Grebe is a monomorphic species I believe the sexes were distinguishable by the male’s brighter plumage, thicker neck, and stockier head.
COURTSHIP
On 5 occasions (23 June-5 July) nesting material was presented by one bird to the other, although unassociated with a nest site or actual nest con- struction (Fig. 2). One bird picked up a piece of vegetation floating on the water, turned and swam to within several centimeters of the other and dropped it. A lily pad was presented once; a bulrush and then some unidentified vege- tation was presented; strands of water milfoil were presented 3 times; and
33
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
I'k;. 1. Location of Kcd-ncckt'd (irchc nests, (iedarville ILiy, Clark Township, Mackinac (iounty, Michigan.
unidentified vegetation was presented once. During a fifth presentation l)oth l)irds simultaneously presented vegetation to the other and then turned away. Immediately after one presentation both Birds called in unison and after another the birds turned tail-to-tail (bodies almost touching) and simul- taneously dipped their hills and shook their heads.
("ourtshii) observations were few, brief, and involved only 3 (Weed Tricks, Head Shaking, and d Timing Away) of the many jjostures and displays given by Wohus (1B6I) as part of the Ked-necked Grebe’s courting repertoire. I believe most of the courtship activities occurred prior to my first sighting the pair on 20 June and possibly some occurred even before their arrival in the marsh. Storer (1060) observed courtship behavior in the Horned Grebe (Padiceps niiritus) along its migration route and suspected it also occurred on its wintering grounds. Bent (1010) and McAllister (1058) wrote that Eared Grebes (Podiceps nigricoUis ) appeared mated on their arrival in the spring, however, McAllister ( 1058) further noted that they may change mates on the breeding grounds. Although I never saw the birds on the first nest (found on 16 June) its presence also suggested that all observations were of a renesting attempt and jirohahly courtship and jiair bond formation initially occurred in M ay.
Chamberlin • RED-NECKED GREBE IN MICHIGAN
35
Brooding ''
Hatching
Incubation
Egg Laying —
Copulation Nest Building Courtship
^ ■ r ■■ ■ ' » ■ I ■ ■ ■ ■ I ' ■ - I > ■ ■ ■ I II ■ ■ I > ■ . ■ I I < t • • • I
25 30 5 10 15 20 25 30 5 10 15 30
June July August Sept.
Fig. 2. Duration of breeding activities of the Red-necked Grebe, 23 June to 29 August 1975.
NEST BUILDING
On 25 occasions (23 June-12 July) I observed nest construction. Obser- vations ranged from 1-90 min duration (Fig. 2). The very brief periods of nest building ( 1-4 min ) appeared to have more significance as post-copula- tory behavior than actual nest construction. The mean duration of nest building bouts, excluding those occurring immediately after copulation, was 21 min.
Nest building was observed at 9 locations which were from 2-70 m apart (Fig. 1). On several occasions the 2 closest nests were worked on simul- taneously. The number of days each nest site was attended by the pair is depicted in Fig. 3. The construction of numerous nests apparently is not un- common. Speirs et al. ( 1964) recorded 7 nests built by one pair of Red- necks on Lake Ontario.
The nest site appeared to be chosen by the male either by poking at the future site with his bill, by starting to carry nest materials to a particular spot, or by Invitation. On one occasion the male left the female on a nest site they had been working on for 4 days and had copulated on, swam 6 m to an- other clump of cattails and assumed the Inviting posture ( i.e. lying flat with neck outstretched and low and the bill pointed forward and almost touching the water). The female called several times but the male did not move. After I min the female joined the male and both began building at the new site.
All nest sites were among the bulrushes and on floating clumps ( less than I m in diameter) of cattail roots and stems. Bulrushes, water milfoil, and lily pads were incorporated into the nest. These materials were collected within approximately a 5-m radius of the nest. Bulrush stems were picked up
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THE WILSON BULLETIN • VoL 89, No. 1, March 1977
9
8
7
6
CO
^ 5
^ A (p 4
Z
3-
2-
20 25
June
30
5 10
July
Fig. 3. Duration of nest building aetiviti(‘s at 9 nest sites, 23 June to 12 July 1975.
singly and carried crosswise in the bill. Water milfoil was obtained during a brief dive. Lily pads were half-carried, half-dragged through the water and lifted onto the nest.
Nest construction appeared to involve 2 steps. J'irst, bulrushes, lily pads, and some water milfoil were placed on a clumi) of cattails. This provided a floating i)latform callable of supporting eggs and an incubating bird. Second, a simple depression was formed by one l>ird on the nest receiving materials (almost exclusively suhmergent vegetation brought up from the bottom I from its mate and pulling these around itself into a low rim. Most of the rim was constructed during the first 2 days of incubation.
Nest building was performed by both birds, although the male was the principal builder during the early stages and was observed vigorously piling vegetation on the nest while the female swam hack and forth, rested, or preened a couple meters away. Later, the male frequently carried materials to the nest where the female, on the nest, arranged them around herself. As the day of the laying of the first egg approached both l)irds were often simultaneously involved in the nest building and on one occasion they worked together con- tinuously for 74 min. On several occasions I saw the female building alone.
During the nest building period the birds rarely approached the nest site alone, although one freiiuently departed before the other. When one finished foraging before the other, it called, preened, and waited until the other joined it. Only when the birds were together did they cautiously return to the nest site several body lengths apart.
The nest in which the eggs were ultimately laid was the seventh nest begun
Chamher/in • RED-NECKED (DiEBE IN MICHIGAN
37
by the pair ( excluding the nest with the 7 eggs j and was among the bulrushes at the edge of the inner open water area in 1.1 m of water (Fig. Ij. It had an inside diameter of 15-16 cm and an outside diameter of 38-42 cm. The depth of the depression was 2.5 cm and the top of the rim was only 5 cm above the water level. The first nest (found on 16 June) was floating in 0.5 m of water, 25 m from the shore, and only 30 m from a road. I suspect it became detached at its anchorage, drifted into the shore, and was consequently aban- doned. The nest was a sodden mass of bulrushes and water milfoil with a 33- cm outside diameter above water and a 61-cm diameter under water. The top of the nest was 6 cm above water and the depression containing the eggs was 15 cm in diameter.
COPULATION
I observed copulation 6 times from 30 June to 11 July (Fig. 2). The pro- cedure for all copulations was essentially the same: (1) The female climbed onto the nest platform and Invited. On 2 occasions the female uttered a faint, plaintive call. (2) Within 0.5-2.0 min the male mounted the female and copu- lated, while on the nest. Copulation was 3-7 sec in duration and accompanied each time by the copulating call ( “Rattern” ) described by Wobus (1964). (3) Immediately after copulation tbe male walked over the female’s head and shoulders and into the water at which time both birds raised their heads. This was followed by (4) Head Shaking by one or both birds or both birds. Slow Swaying ( “Wegsehen” ) , and (5) either both birds preened briefly or the female preened while the male briefly collected nest material.
My observations were in accord with those of Wobus (1960, 1964). The faint vocalization of the female in the Inviting posture may correspond with the platform call of the Horned Grebe described by Storer (1969).
EGG LAYING
Three eggs were laid. The first egg was laid on 11 July and had a bluish matrix which became, by the time it hatched, dark brown due to staining from wet vegetation. I don’t know the exact dates of the laying of the second and third eggs. Wobus (1964) found that the average clutch size for July-nesting birds was 2.5.
! INCUBATION
I observed incubation for 151 h and 39 min from 11 July to 9 August (Fig.
' 2). Incubation was shared by the sexes, the male incubating 41% of the time
and the female 59%. For comparison I divided the day into two 8-hour I blocks; one representing mid-day (10:00 to 17:59) and the other morning ! and evening (06:00 to 09:59 and 18:00 to 22:00). In the morning and evening intervals the female incubated 65% of tbe time, whereas during the
38
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
o oooooooooooooooo o oooooooooooooooo
O h-OOOO— CVirO^LOOl^OOOO — C\j O OOO— — — — — — — — — — C\JC\JC0
Time Of Day
Fig. 4. Average percent of time spent on the nest by sex and time of day. Each time block represents a minimum of 5 liours of observation.
middle of the day she was on the nest only 41%. During all observations be- fore sunrise and after sunset the female was on the nest, suggesting that she lierformed most of the night-time incubating. From day to day each sex tended to incubate at approximately the same times (Fig. 4).
As the birds adjusted to the incubation routine, the average duration of their individual attentive i)eriods increased. The average attentive period lasted 87 min during the first week but increased to 132 min during the second with a corresponding decrease in the number of periods per day from 10 to 7. File last few days of incubation were similar to the first days of incubation in that the mean duration of the attentive periods decreased to 78 min and the freciuency of change-overs increased back up to 10 per day. Wobus ( 1961 1 found the average duration of attentiveness to be 1-2 h.
An all-day observation on the third day of incubation revealed that incuba- tion was continuous, or nearly so, during the egg-laying and early incubation periods. During the first 3 days incubation was infreiiuently interrupted by brief periods of nest building and coi)ulation but from the fourth day on the eggs were very rarely and briefly left unattended.
During the nest reliefs, or change-overs, the returning bird’s behavior ap- peared dependent not only on the strength of its own urge to incubate but also on the incubating lord’s readiness or reluctance to leave the nest. Occa- sionally during extremely hot weather (e.g. 32°C) and after unusually long periods of attentiveness (e.g. 3M.'-4 h), the mere presence of the returning
Chamberlin • RED-NECKED GKERE IN MICF1K;AN
39
bird was sufficient to induce the incubating l)ird to leave the nest. Head Shaking by tbe returning bird was the dominant component of nest reliefs and in most cases induced its mate to leave the nest. Head Shaking was part of 49 (68%) of the 72 nest reliefs I observed. During the first week’s nest reliefs the returning bird Head Shook as many as 6 times and often the incu- bating bird also participated in Head Shaking. By the second week Head Shaking was primarily by the returning bird and only done once or twice per nest relief. If Head Shaking failed the returning bird often sat next to the nest (usually to the rear of the incubating bird) and performed comfort movements or poked at the nest for several min. When its mate still remained on the nest the returning bird made brief nest building actions. Twice the male simply “gave up” after these attempts and left for a while; once the female jumped onto the nest forcing the male off. Herring Gulls {Larus argen- tatus) demonstrate a similar behavioral progression during nest reliefs (Tin- bergen 1960, pers. observ.j. The returning gull’s inducements ranged from its mere presence on the territory, to Mewing, to Choking, to bringing nest material to the nest, to physically evicting its mate from the nest.
Three times during the first 2 days of incubation the female, on leaving the nest. Reared and Wing Quivered, thus “coaxing” the male onto the nest. These 3 occurrences were the only times I saw Wing Quivering. Storer ( 1969) discussed this display as the most intense form of soliciting. The Inviting posture, a milder form of soliciting, was assumed by the incubating bird as its returning mate swam towards the nest. Inviting remained as part of the nest relief pattern through the seventh day, after which I no longer saw it. Prior to egg laying the nest platform had been used primarily as a copulation platform. Thus the occurrence of soliciting postures during the first nest reliefs suggested they were a carry-over from copulation, and possibly such actions on the part of the female encouraged the male’s transition to incu- bating behavior.
Also during the first 2 days of incubation, nest reliefs were twice initiated by the female (as the returning bird ) carrying nest material to the nest but not depositing it thereon. Instead she swam back and forth in front of the male as if to entice him off the nest by an activity in which he had, until recently, been vigorously involved. Carrying nest material, as well as solicit- ing, may have reflected the ambivalence present in the birds as they changed from one behavior pattern to another. I saw none of these activities as part of nest reliefs after the first week of incubation. The nest reliefs gradually be- came less complex ( i.e. fewer movements and postures ) as various components were “phased out.”
Several change-overs occurred in which I saw none of the usual cues, but rather they appeared to be initiated by impatience, rain damage to the nest,
40
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
or disturbances. On the third evening of incubation the male abandoned the nest after a long period of incubating. Both birds returned 27 min later and the female climbed onto the nest. Once after a 44-min downpour, the male slid off the nest and began nest repairing. The female appeared 2 min later and climbed onto the temporarily abandoned nest. The male continued to repair the nest for an additional 18 min. Change-overs occurred twice when the incubating bird was frightened off the nest by fishing boats and once by one of my nest checks.
The birds approached the nest cautiously from the open water, rather than through the bulrushes. During the first 10 days the birds swam on the surface to the nest, usually with considerable head-bobbing movements. Starting on the eleventh day the female approached the nest underwater, diving from 20-30 m away. Seven times when the female surfaced right next to the nest and face to face with the incubating male they both raised their necks up as tall as possible, crest plumes erect, and called loudly and simultaneously. The same display occurred twice when the incubating bird left the nest at the sight of the returning mate. The birds displayed and called as they swam towards each other, and turned face to face as they passed. This mutual upright l)osturing and vocalizing ai)parently was a greeting of mate recognition. According to Storer ( i>ers. comm. I this vocalizing corresponds to the Triumph Ceremony of the Horned Grebe and the greeting trills of the Pied-billed { Podilyinbus podiceps) and Least grebes iPodiceps dominicus) .
After each nest relief the departing bird spent 3-20 min (mean = 8) preen- ing before it swam out through the bulrushes to forage in the channel. The male was markedly more vocal than the female and often called during his returns to the nest, although by the 6th day his returns had become silent. I rarely heard vocalizing in the immediate vicinity of the nest after the first week of incubation, except during the aforementioned change-overs and after disturbances.
LATE IX CUB ATI ON
During the last 5 days of incubation, nest building was freciuent and occurred in conjunction with 7 of the 18 nest reliefs of this period. Except for one instance of nest repair, I had not observed extensive nest building since the second day of incubation when rim construction was completed. Since token nest building was a strong nest relief cue, such behavior may have indicated strong drives to incubate or possibly it was displacement activity reflecting frustrations caused by the sounds of chicks within the eggs and internal changes in the birds’ drives from incubating behavior to broodiness.
Two days before hatching a new behavior, which I call Lunging, was incorporated into 6 of the 8 observed nest reliefs. Lunging consisted of a
Chamberlin • RED-NECKED GREBE IN MICHIGAN
41
slabbing motion of the bill towards the inculiating bird’s back and was made by the returning bird as it sat next to the nest. Lunging was apparently an intention movement of feeding the young, during which the adult presents food in the bill to the chicks on the other parent’s hack.
Several days prior to hatching the incubating bird freciuently stood up and either looked down at the eggs or rearranged the nest material around them. Such behavior was most likely stimulated by chick sounds within the eggs. During the earlier days of incubation the birds rarely stood up once comfortably settled on the eggs. During these last days of incubation the non-incubating bird spent considerably more time loafing in the vicinity of the nest than it had before and the birds started approaching the nest through the bulrushes, which they had not done previously.
HATCHING
The 3 eggs hatched on 6, 7, and 9 August I Fig. 2). Since the first egg was laid on 11 July, and incubation began on the same day and was con- tinuous throughout, the incubation period for the first egg was 26 days. Bent ( 1919 ) determined the period of incubation to be 22-23 days for eggs he hatched in an incubator. The eggs’ constant contact with wet vegetation and the possibility that, although the grebes were continuously on the eggs starting with the laying of the first egg, heat transfer may not have yet been complete might have accounted for the longer incubation period in the wild. Wobus ( 1964) gave the average incubation period as 23 days but added that it is often longer due to cold weather and/or disturbances.
Hatching occurred in the mid-morning. From 09:49 to 10:24 on the morning the first egg hatched the incubating bird showed considerable un- easiness and stood and looked down at the eggs 6 times. The next day the second egg was intact at 07:30 and the second chick was first observed crawling out from under the incubating adult at 11:45. Two days later at 07:48 the third egg was still intact but during the change-over at 12:38 1 saw the chick in the bottom of the nest while the other 2 were in the water. Before settling onto the nest the male picked up the egg shell and dropped it over the rim of the nest.
BROODING
The chicks were brooded on the parents’ backs under their wings when the adults were on the nest as well as on the water. This undoubtedly had survival value considering the cold, wet state of the nest and the presence of acjuatic predators such as the northern pike iEsox lucius). Brooding was performed by both sexes and brooding periods ranged from 57-162 min (mean = 119). During change-overs on the nest the brooding bird stood up.
42
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
spread its wings, and shook the chicks off its back and into the nest. After the adult dismounted, its mate climbed onto the nest and raised its wings 4-5 cm off its back allowing the chicks to crawl up and under. During change- overs on the water the brooding bird raised itself into a nearly vertical position and shook off the chicks by wing-flapping. At 3 weeks of age the chicks were no longer brooded on the adults’ backs.
After the hatching of the third and last chick on 9 August the nest was used during most of the following day, from 09:29 to 20:52, and then abandoned. I never saw the birds using the nest after 10 August.
FEEDING THE CHICKS
Both parents fed the chicks. During the first week the chicks were fed 2- to 5-cm minnows, small unidentified items ( probably insects), and an occasional feather. The age of the first chick I observed being fed a feather was 2 days. Wobus ( 1964) observed chicks being fed feathers, as well as insect larvae, during their first day of life. After the first week the food appeared to be almost exclusively fish in the 4-10 cm range. During the third week the fish were noticeably heavier bodied and once a medium-sized crayfish was fed to one of the chicks.
As the parents swam towards the chicks with a fish they repeatedly dipped the fish in the w ater and appeared to be manipulating and pinching it in their bills, as described by Sim (1901 1 . I bis probably killed and softened the fish and made swallowing and digestion easier. Ihe food was held in the tip of the bill and presented to tbe clucks while they were on the other parent’s back. I he chicks freciuently dropped the minnows during the first several days’ feedings but the parents picked u]) the dropped minnow and presented it repeat- edly until the chick finally got it headfirst into its mouth. I he brooding parent frecpiently picked up any dropped items and fed the chicks on its own back. By the second week the chicks were fed on the water where they persistently begged for food and swam out to meet the parents each time they returned w ith food, and occasionally even pursued their parents underw ater.
Feeding periods during the first week ranged from 23-113 min ( mean = 75) with a mean of 9 feedings per period (Table 1). Feeding intervals ( i.e. the time between individual feedings I ranged from 1-32 min (mean = 8). A 28- to 129-min loafing period (mean = 58 I, during which the non-brooding bird loafed and/or foraged for itself, immediately preceded or followed each change-over. During the second week feeding periods were one-third as long as during the first week while the number of feedings per period more than doubled due to the 6-fold reduction in the length of time between feedings (Table 1 ). Thus the chicks’ growing demand for food was met by decreasing the time interval between feedings.
Chamberlin • KED-NECKEI) GKEliE IN MICHIGAN
43
|
A Weekly |
Gomparison |
Table 1 OF Feeding and |
Loafing Periods |
|
|
Week |
||||
|
1st |
2nd |
3rd |
||
|
Duration of feeding period |
s (min) |
23-113 |
16-34 |
4-27 |
|
(75)=^ |
(25) |
(17) |
||
|
Number of feedings per period |
2-15 |
10-31 |
7-32 |
|
|
(9) |
(19) |
(16) |
||
|
Feeding intervals (min) |
1-32 |
0.5-12 |
0.25-7 |
|
|
(8) |
(1.4) |
(1.1) |
||
|
Duration of loafing period |
s ( min) |
28-129 |
32-64 |
23-39 |
|
(58) |
(50) |
(31) |
||
|
Mean number of feedings |
per hour |
3.5 |
12.5 |
18.0 |
* Means are given in parentheses.
The mean duration of the feeding periods and feeding intervals continued to decrease through the third week ( Table 1) . The reduced number of feedings per period simply reflected the shorter duration of the periods. The very short time intervals such as 0.25 min between some feedings were probably the result of both parents simultaneously feeding the chicks. Short time intervals probably also occurred when the birds found their prey concentrated in large schools. Once when the chicks were fed 74 times in a 25-min period, every fish appeared to be the same size (4-5 cm) and while the birds fished they moved steadily along as if following a school. During the third week the mean duration of the loafing periods was 47% and 38% shorter than during the first and second weeks, respectively. Thus although the feeding periods were shorter they were also more frequent, as indicated by the reduced amount of time the adults spent loafing in between. As the chicks grew the mean number of feedings per hour increased steadily from 3.5 the first week, to 12.5 during the second week, to 18.0 during the third week (Table 1).
1 last observed the family on 28 September 3.5 km from the nest. The 3 chicks, at 51, 53, and 54 days of age, were still being fed by both parents. According to Wobus ( 1964) the family bonds break up after 8 to 10 weeks.
INTERSPECIFIC RELATIONS
Red-winged Blackbirds (Agelaius phoeniceus) were in frequent attendance of the grebes’ nest building. After the grebes’ departure from a nest platform, the Red-wings immediately dropped down to the nest and appeared to be snatching up insects, probably brought up with or attracted to the wet vegeta-
44
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
lion. Deusing (1939) watched a Longd3illed Marsh Wren (Telmatodytes palustris) catching insects on a Piechbilled Grebe’s nest.
The incubating grebes tolerated the passing and activities of other marsh Hesters such as the Piedd^illed Grebe, American Coot [Fulica americana) , Black Tern (Chlidonias niger) and Red-winged Blackbird near the nest. The several species of ducks (Anas platyrhynchos, A. rubripes, A. discors, and Aix S])onsa) common in the marsh were tolerated as they fed near the nest prior to the hatching of the grebe chicks.
The grebes assumed a defensive posture (neck and head upstretched with the bill directed towards the cause of the alarm) whenever Herring Gulls flew low and noisily over the nest. Once 2 immature Ring-billed Gulls ( Larus delawarensis I dived at one of the 3- week-old chicks and stimulated a half- running, half-flying attack by both adults. These same 2 gulls also tried to rob the male grebe of a crayfish, but only forced him to dive out of their reach. A female Marsh Hawk [Circus cyaneus) , gliding only 4-5 m directly over the nest, caused the male, with the chicks on his back, to leave the nest.
At hatching the male became strikingly territorial towards other species. On one occasion the male drove off 2 immature Pied-billed Grebes that were foraging at least 40 m from the nest. The male approached one of the Pied-hills in a very i)ronounced threat attitude, hunched very low on the water. The male dived and in the same instant the Pied-bill half-ran, half-flew 3-4 m across the water. The male surfaced and continued to i)ursue the Pied-hill in a threat attitude. When the male dived a second time the Pied-bill flew off.
I he male then turned his attention to the other young grebe and with a similar secpience of actions drove it away. According to Storer (1967) grebes seem to fear underwater attacks and so do not remain on the surface if an aggressive grebe dives.
In aggressive situations diving is ‘‘understood” as a threat between different s|)ecies, and even genera, of grebes but apparently not between higher taxonomic groups. W hen the male approached a Mallard feeding 10 m from the nest in a threat attitude, the duck continued feeding, and as the male dived the duck remained oblivious to the grebe’s actions. However, several seconds after the male dived the Mallard si)iang into the air and, (juacking loudly, flew off. Apparently the duck did not “interpret” the dive as a threat and so fled only after (presumably) being physically attacked from underwater.
I he pair’s interspecific territoriality appeared to be in defense of the brood and not the nest site or any fixed area of the marsh. This was suggested by the rarity of agonistic behavior prior to and during incubation and the sudden aggressiveness at hatching. Also, their aggressiveness extended far beyond the nest site and even after the nest had been abandoned. As the brood moved so did the territory. Both adults frequently chased away Pied-billed Grebes,
Chamber/in • RED-NECKED GREBE IN MICHIGAN
45
Mallards, a female goldeneye {Bucephala clangula), and a Great Blue Heron [Ardea herodias) that came close to the brood as they traveled along the marshy shorelines of the channel.
Twice during the hatching period a muskrat (Ondatra zihethica) swimming close to the nest was threatened by the male. When the muskrat dived the grebe immediately followed. Shortly, the muskrat surfaced and continued on its way and the male returned to the nest. The only other interaction w ith muskrats was that the abandonment of nest site #4, after 4 days of use by the grebes, coincided with muskrats starting to use it for one of their feeding platforms.
SUMMARY
A pair of Red-necked Grel)es iPodiceps grisegena) was studied in a northern Lake Huron marsh from 20 June to 29 August 1975. Observations were of a renesting attempt and courtship l)ehavior was brief and infreciuent. Copulation occurred on the nest platform from shortly after nest building began into the egg-laying period.
The nest site was selected by the male although both sexes built the nest. The pair constructed 9 nest platforms, one of which ultimately became the nest in which 3 eggs were laid. Incubation began with the laying of the first egg and both sexes incubated, although the male incubated more during the mid-day and the female more in the morning and evening hours. Nest reliefs were initiated primarily by Head Shaking by the returning bird.
The first egg had a 26-day incubation period. Hatching occurred in mid-morning. The nest was abandoned 2 days after the hatching of the last chick. The chicks were brooded on the adults’ backs under their wings. Both sexes brooded and fed the young. Food items consisted of minnows, crayfish, and probably insects. The mean number of feedings per hour increased from 3.5 to 12.5 to 18.0 during the first, second, and third weeks, respectively. Three chicks were successfully raised to over 7 weeks of age.
ACKNOWLEDGMENTS
I thank Dr. Robert W. Storer for his helpful comments and Dr. Nicholas L. Cuthbert for proofreading an early draft of the manuscript. Appreciation is also extended to Jim Hammers for translating several articles.
LITERATURE CITED
American Ornithologists’ Union. 1957. Check-list of North American birds, 5th ed. Baltimore, Am. Ornithol. Union.
Bent, A. C. 1919. Life histories of North American diving birds. U.S. Natl. Mus. Bull. 107.
Deusing, M. 1939. Nesting habits of the Pied-billed Grebe. Auk 56:367-373.
Jones, S. P. 1938. Holboell’s Grebe and American Brant in Wisconsin. Auk 55:666. Mahan, H. D. 1963. Michigan bird survey, summer, 1962. Jack-Pine Warbler 41:75. McAllister, N. 1958. Courtship, hostile behavior, nest-estahlishment and egg laying in the Eared Grebe iPodiceps caspiciis) . Auk 75:290-311.
Sim, R. J. 1904. Notes on the Holboell (irebe (Colymbus holboellii). Wilson Bull. 16:67-74.
46
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
SpEiris, J. M., G. W. North, and J. A. Crosby. 1944. Holhoell’s Grebe nesting in southern Ontario. Wilson Bull. 56:206-208.
Storp:r, R. W. 1967. Observations on Rolland’s Grebe. Hornero 10:339-350.
. 1969. The behavior of the Horned Grebe in spring. Condor 71:180-205.
Tinbergen, N. 1960. The Herring Gull's world. Doubleday & Co., Inc., Garden City. N.Y.
WoBUS, U. i960. Nestbau und balz des Rotbalstaucbers. Die Vogelwelt 81:61-62.
. 1964. Der Rotbalstaucher (Podiceps grisegena Boddaert ) . A. Ziemsen, Witten- berg Lutherstadt.
Zimmerman, D. A. and J. Van Tyne. 1959. A distributional check-list of the birds of Michigan. Univ. of Mich. Mus. Zook, Ann Arbor.
INTEltLOCHEN AUTS ACADEMY, INTERLOCIIEN, MI 49643. ACCEPTED 20 JAN. 1976.
NEW LIFE MP:MBEK
Mr. Hub(‘rt B. Ziunickow is a new life member of the Wilson Ornithological Society. Mr. Zernickow is an Advisory Systems Engim*(*r with the IBM (Corpora- tion at Lansing, Michigan. His ornitho- logical interests are primarily as an (d>- server and photographer, and he has a special interest in owls. .Mr. Zernickow is a member of the AOU and is presently presi- dent of the Michigan Audubon Society. He is married and his wife, Norene. shares bis enthusiasm for birding. In addition to his ornithological interests. Mr. Znnickow •‘iijoys botany, hiking, ami canoeing.
GROWTH AND DEVELOPMENT OE THE Pr.AIN CHACHALACA IN SOUTH TEXAS
Wayne R. Mauion
Growth and development of many game birds have received thorough in- vestigation, but this is not true for the Plain Chachalaca ( Ortalis vetula ) . I studied growth and development of chachalacas as part of a larger study (Marion 1974) of the ecology of this species in Texas.
METHODS
Research was conducted from January 1971 to August 1972 at Santa Ana National Wild- life Refuge, and 3 other study areas in the Lower Rio Grande Valley, Hidalgo and Starr counties, Texas. Data reported in this paper were obtained from captive birds, live- trapped birds, and collected specimens.
Captive chachalacas were reared in 2 weld-wire pens at Santa Ana National Wildlife Refuge headcfuarters. An attempt was made to keep the pens as natural as possible; they included several small trees and additional plant materials which provided cover, shade, and sites for perching and nesting. One pen (4.6 X 3.0 X 2.4 m) contained 3 adult birds fl^, 2$ $). Another slightly larger pen (6.1 X 7.2 X 2.4 m) contained 7-9 immature birds (2-3 9 9, remainder^ ^). Captive juveniles were hatched in an incubator from eggs collected from 4 nests during 1971. Fresh water and commercial foods were pro- vided ad libitum. Captive birds were fed commercial starter, grower, and maintenance rations, corresponding to stage of maturation. Natural foods were fre(juently provided to supplement commercial foods.
Chachalacas were live-trapped at the main study area in 25 X 50 mm mesh weld-wire traps (1.2 X 1.2 X 0.6 m) with funnel entrances similar to those described by Taber and Cowan (1963:261). Twelve traps were operated at randomly selected sites during late winter and early spring of both years of this study. Traps were baited with fresh cabbage and grain sorghum placed within the enclosure and near the funnel entrances. To avoid excessive stress on handled birds due to overheating, trapping was restricted to the cool morning and evening hours. Trapped birds were marked with aluminum hands and colored leg streamers for subsequent individual field recognition.
Chachalaca specimens w^ere shot in a nonselective manner on all study areas between September 1971 and August 1972. Many birds were collected in the morning and evening when they were more active; fewer were collected at midday. Data from carcasses of chachalacas found dead during this study also were recorded.
There is no obvious sexual dimorphism in this species, hut all birds handled were sexed using at least 1 of 4 known methods. Two of these methods, convenient for sexing live birds, were related to the presence or absence of a looped trachea. The adult male has a trachea lengthened by a loop which is easily felt between ventral musculature of the breast and the skin; this looped structure is lacking in young males and females (Merrill 1878). During this study, inspection of gonads of sacrificed birds verified this sexual difference in tracheal development of adult birds. Determination of the presence or absence of the tracheal looj) by feeling the breast was the major technicjue used in sexing older juveniles and adults.
47
48
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
Generally the longer and wider the trachea, the deeper the bird’s voice; the shorter and narrower the trachea, the higher the voice. Adult males have a longer trachea and their voice is a full octave lower than that of females and young males (Sutton 1951:127). The pitch of the voice was a second method used in sex determination when live birds were heard calling.
Chachalaca males have a penis which can he readily observed by cloacal examination, hut this teehniciue was rarely used because it re(|uired more handling of birds than the previously described tracheal loop method. All collected specimens were sexed by exam- ination of the gonads.
Definitive aging criteria for chachalacas have apparently not been reported. I recorded tracheal loop lengths, measured externally from the distal portion of the loop to the |)oint of entry into the thorax, for use as aging criteria for male birds. Total length and diameter (at each end and near the middle) of the trachea of all collected specimens were measured to determine sex and age differences. Vernier calipers, permitting readings to the nearest O.I mm, were used in measuring tracheal diameters.
Postal scales were used to measure total body weight to the nearest 0.5 ounce. These data were later converted to ecpiivalent values in grams. Several standard length measure- ments for birds, including total body, wing chord, tail, exposed cuhnen, tarsus ( tarso- metatarsus), middle toe, and total extent of wings, as described by Baldwin et al. (1931) and Pettingill (1946:323-325), were taken to the nearest 1 mm using a pair of dividers and a ruler. These measur(*ments were recorded for captive (at intervals of 1 month or less), live-trapped, and colIectc<l birds. Plumages and molt patterns of all handled birds also were examined.
KESULTS AM) DISCUSSION
I live-trapped, color-marked, and released 222 chachalacas (144 in 1971, 78 in 1972). den of the marked birds from 1971 were recaptured in 1972. An additional 61 chachalacas (82 in each year) were sacrificed at the 4 study areas, with the majority from Santa Ana National Wildlife Refuge.
Sex ami a^e detennination. — dTacheal development in chachalacas was successfully used to determine sexual differences and to distinguish juvenile from adult males. Measurements of total tracheal length could not he taken externally and were all obtained from sacrificed birds. Fhe mean total length of the trachea for 18 sacrificed adult males was 829.8 ± 20.6 mm (range 2 L5-885 mm), more than twice the average tracheal length for 28 sacrificed adult females (111.7 ± 10.8 mm, range 121-168 mm). Measurements of maximum diameter of the trachea, taken near each end and at the middle, also showed sexual differences. Mean tracheal diameters were significantly larger for adult males than for adult females at the anterior (upper) end (t = 8.5, P < 0.01), near the middle (t = 4.5, P < 0.01), and at the posterior (lower) end (t = 8.5, P < 0.01).
The tracheal loop began to develoj) in juvenile males at about 8 weeks of age and was easily felt on the anterior breast at 10 weeks. Tracheal loop lengths were measured either internally or externally; the former measure- ment retpiired that birds he sacrificed, whereas, the latter was used without
Marion • CHACHALACA GROWTH
49
harming living birds. Length of the tracheal loop was measured by both methods from the distal end of the loop to the point of entry into the thorax. For comparison, loops of 19 collected adult males were measured using both techniques. Mean external measurements were slightly larger than mean internal measurements (73.0 ± 3.6 mm and 70.4 ± 4.6 mm, respectively). These differences, however, were not significant ( t == 1.9, P > 0.05 ) .
External tracheal loop measurements were recorded for male chachalacas. Mean loop length of captive juveniles was 17.5 ±7.1 mm (range 13-23) at 9 weeks of age (N = 2) and 30.0 ± 7.2 mm (range 34—38 mm) at 10 weeks of age (N = 3). The tracheal loop of captive juveniles elongated slower than other body parts; measurements began overlapping those of adults when young males were approximately 9 months old (Fig. 1).
Tracheal loop development was apparently slower in wild juveniles than in captive juveniles. Mean loop lengths for wild juveniles handled during February (N = 9) and March 1972 (N == 4) were 51.2 ± 4.4 mm (range 46-60 mm) and 54.1 ± 3.5 mm (range 49-57 mm), respectively. The aver- age loop lengths of 5 captive juveniles in February and March were 62.1 ± 4.3 mm (range 58-67 mm) and 64.8 ± 2.4 mm (range 63-68 mm), respec- tively. These measurements for captive juveniles were significantly larger than those for wild juveniles in February (t = 4.4, P < 0.01) and in March (t = 5.5, P < 0.01). Several variables, all related to the exact age of wild (unknown age) vs. captive (known age) juveniles, may he responsible for this difference hut the major cause remains unknown.
These data generally indicate that wild juveniles may he distinguished from adults using tracheal loop lengths until at least 9 months of age. By 1 year, tracheal loop development was nearly complete and differences among males were subtle. Further increases in tracheal loop length as males aged were apparently minor. Four banded males, known to be more than 5 years old, had an average loop length of 78.0 ± 3.1 mm (range 75-82 mm). This mean value, although slightly larger than the average loop length of 44 other adult males (74.2 ± 4.4 mm, range 67-87 mm), was not significantly larger (t = 1.7, P > 0.05).
Significance of color of the upper mandible was investigated as an indi- cator of age. Presence or absence of a dark tip on the upper mandible was recorded for 17 juvenile males (tracheal loop partially developed) and 17 adult males (tracheal loop well developed) handled between December 1971 and March 1972. Upper mandibles of all 17 juvenile males had dark tips. Only 2 of 17 (11.8%) adult males had dark-tipped upper mandibles. The remaining 15 ( 88.2%) had uniformly colored ( blue horn ) bills. These results indicate that dark markings near the tip of the upper mandible are characteristic of juvenile birds.
90
80
70
60
50
40
30
20
10
0
ir(*mei
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
N (Adult Males) =
N (Juvenile Males) =
2355555555555 5 5 3
I I
A S
I 2 3
T‘ T""'l I I I I I "
ONDJFMAM
4 5 6 7 8 9 10 II
AGE (MONTHS)
raclieal loop lengtlis of wild adult and captive juvenile cliachalacas. External s were taken and data presented are means (rtSD) and ranges.
Marion • CHACHALACA GROWTH
SI
Strength of the lower mandible varies with age in many gallinaceous birds. Generally, if a dead bird is supported only by the lower mandible and it breaks, the bird is a juvenile; if the lower mandible does not break, it is an adult (Leopold 1933:166, Taber 1963:134). Lower mandible strength was determined for males of known age ( by tracheal loop development ) collected between December 1971 and March 1972. When subjected to the “lower mandible test,” all 4 juvenile males had mandibles that broke. Each of the 10 adults tested had a lower mandible that supported the bird’s weight. Strength of the lower mandible seems to be a valid technicjue for dis- tinguishing juvenile and adult chachalacas.
Males of known age (determined by tracheal loop development) handled between December 1971 and March 1972 were used to investigate leg color differences between juveniles and adults. Of 18 juveniles, 8 (44.4%) had legs that were slightly orange. The others had darker ( blue horn ) legs. Of 22 adults, only 2 (9.1%) had slightly orange legs. A significant (P < 0.05) chi square value of 6.6 indicates that orange legs are more tyi)ical for juve- niles than for adults.
Age determination in gallinaceous birds commonly involves plumage char- acteristics (Taber 1963:128), however, plumage differences between juvenile and adult chachalacas diminish rapidly as young birds mature. At 2 or 3 months of age, slight differences exist in width and shape of flight feathers. Rectrices and remiges of juveniles are relatively narrow and pointed, while those of adults are broad and rounded. Most of these plumage differences are lost with the postjuvenal molt before the juveniles are 6 months old. The outermost juvenal rectrices and primaries, however, may be retained slightly longer.
Growth. — Chachalaca chicks are precocial and leave the nest within a few hours after hatching. Chicks are very active and agile in climbing through trees and shrubs within a few days of hatching and are able to jump and fly at least 1.3 m at 6 days of age.
Weights and measurements of wild adults (males and females) and captive juveniles (1 week and 1 month old) are presented in Table 1. Adult males averaged significantly (P < 0.01) larger than adult females in weight, total length, wingspan or extent, wing chord, tail length, exposed culmen length, tarsus length, and middle toe length. Although statistical comparisons of mean values indicate that adult males average larger than adult females, much overlap exists in the ranges of these measurements. These overlapping values reflect subtle sexual differences in size which are not easily recognized in the field.
Mean adult weights were highest during October and November (631 ± 87 g and 646 ± 97 g, respectively), but were relatively constant during other
1
52
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
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WEIGHT (grams)
Marlon • CHACHALACA GROWTH
53
800-
700-
600-
500-
400-
300-
200-
100-
N ( Adults) =
3 4 8
7 2 5 23 12 65 73
ADULTS ^
|
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||
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CAPTIVE JUVENILES
N (Juveniles) =
998868777774 I I I r I I I I I 1
ASONDJ FMAM
I 234 5678 9 10 II
AGE (MONTHS)
Fig. 2. Weights of wild adults and captive juvenile chachalacas. Data presented are monthly means (± SD) and ranges.
51
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
months (Fig. 2). An abundance of natural foods following floods in the fall of 1971 and heavy fat deposition may account for the increased adult weights during October and November. In addition, 11 of the 15 adults measured during these 2 months were males and this may have also contributed to the increase in recorded weights. Average weights of captive juveniles increased rapidly and, at 4 or 5 months of age, approached the 550-600 g level of adults (Fig. 2j. Similarly, hut at only 3 or 4 months of age, captive juveniles ap- proached adult size in total length, extent or wingspan, wing chord, and lengths of the tail, exposed culmen, tarsus and middle toe. Growth rates for wild juveniles may he slightly slower than this due to the disproportionate sex ratio among the captive juveniles favoring the males (and thus, larger size ) .
Plumages and nwltin^. — Kectrices begin to develop at less than 1 week of age and grow rapidly. Initial rectrices are uniformly colored and rather narrow with i)ointed tij)s which are easily worn and broken. Remiges (except for the outer 3 primaries I are well developed at hatching and continue rapid development for several weeks after hatching.
At approximately 1 month of age, juvenal i)lumage begins to replace natal plumage and juvenile birds begin to resemble adults. Body feathers of the head ajid neck region are the last to he replaced by drab olive plumage characteristic of adults. I he juvenal plumage stage in this species is rela- tively short. Juvenile chachalacas replace rectrices and remiges during the |)ostjuvenal molt. Postjuvenal molting of rectrices in captive juveniles oc- curred between August and December, when the birds were 2-6 months old. Postjuvenal molting began with intermediate ])airs (Nos. 3 and 4) in each half of the tail and proceeded both iiiward and outward until all pairs were replaced. Pair No. 3 was usually replaced slightly before i)air No. 4, hut both pairs were replaced in late August when juveniles were 2 months old. Replacement of i)airs 2 and 5 occurred in late September when the birds were 3 months old. Rectrices 1 and 6 were molted over a longer interval; the central pair (No. 1 ) was replaced between September and December and the outer i)air (No. 6) was replaced between September and January.
Observations of wild juveniles handled during si)ring handing operations indicated that outer rectrices are occasionally retained until March. These older rectrices are easily recognized since they lack white tips and are obvi- ously old and worn. Following the postjuvenal molt, rectrices of young birds were white-tipped and both remiges and rectrices were relatively broad with rounded tips (as in adults).
After the breeding season each year, feathers are replaced during the post- nut)tial molt. This molting is gradual and flight is not inhibited. Postnuptial molting of rectrices began as captive juveniles approached 1 year of age and
Marion • CHACHALACA GROWTH
55
the sequence was extremely irregular compared to the post ju venal molt which followed a definite sequence ( 3-4-2-5-1-6 ) . Postnuptial molting of adult rectrices was also irregular, with no obvious pattern or se(iuence. Most rectrices of adults were molted during August and September, but this oc- curred as early as May and as late as December.
Molting of primaries was se(iuential (proximal to distal) for both juvenile and adult birds. Molting observations for remiges of captive juveniles were not recorded prior to the age of 4 months. In the first year, captive juveniles replaced the outer 2 primaries (IX and X) during the postju venal molt. In most gallinaceous birds, except Ring-Necked Pheasants { Phasianus colchi- cus) , the 2 outer primaries are not replaced during postjuvenal molting (Taber 1963:134).
Captive juveniles began the postnuptial molting of proximal (I and II) primaries in February, 2 months prior to adults and this continued until all distal primaries were replaced in the late summer and fall. Postnuptial molt- ing of adult primaries occurred in an ascending pattern similar to that de- scribed for juveniles. Replacement of proximal (I and II) adult primaries began in April and continued until all distal primaries were replaced in the fall. The majority of primary molting in adults occurred during August and September.
Molting of secondary wing feathers was not as distinctly seciuential as in primary wing feathers. Secondaries of captive juveniles were molted during all months of year and postjuvenal molt was not clearly distinguishable from the postnuptial molt. Likewise, postnuptial molting of secondaries in adults followed no definite pattern, but most were being replaced during August and September.
SUMMARY
Plain Chaclialaca growtli and development were investigated in 1971 and 1972 in the Lower Rio Grande Valley of Texas. Chaclialaca chicks are precocial and growth and development of juveniles is rapid. At 4 or 5 months of age, juveniles resemlde adults and field recognition of differences becomes difficult. Size measurements are valid age criteria only during the summer and fall when juveniles are less than 4 or 5 months old. Differences in tracheal loop development (males), molting of outer primaries and rectrices, color and strength of bills and color of legs are valid criteria for distinguishing juveniles from adults.
Plumage changes also occur rapidly; postjuvenal molting begins in early fall when juveniles are nearly 2 months old. Postjuvenal molting of rectrices follows a definite sequence of pairs (from innermost to outermost) and is usually completed before .Janu- ary of the first year. Juvenal primaries are also molted sequentially from the innermost to the outermost. Postjuvenal molting of secondaries is not distinctly sequential.
Adult rectrices are molted in an irregular pattern during the postnuptial molt ( August and September). Adult primaries are molted in a sequential pattern (innermost to outer- most), but postnuptial molting of adult secondaries follows no definite pattern.
56
THE WILSON BULLETIN • Vol. S9, No. 1, March 1977
ACKi\OWLEDGMENTS
I acknowledge with thanks financial assistance from the Caesar Kleberg Research Program in Wildlife Ecology at Texas University. Sincere thanks go to W. H. Kiel,
Jr. for his advice and encouragement during this study. I am also indebted to K. A. Arnold, J. I). Dodd, T. M. Ferguson, and J. G. Teer for their critical reviews of the manuscript. Agencies granting collecting permits were the Texas Parks and Wildlife Department and the U.S. Fish and Wildlife Service. The latter agency also granted permission to hand and color-mark birds at the main study area. This paper is part of a dissertation in partial fulfillment of requirements of the Ph.D. degree at Texas A&M University. This is Texas Agricultural Experiment Station Technical Article No. 12054.
LITERATURE CITED
Baldwin, S. P., II. C. Ohkkhoi.skr. and L. (L Worley. 1931. Measurements of birds.
Cleveland Mus. Nat. Hist. Sci. Puhl., Vol. 11. Cleveland, Ohio.
Leopold, A. 1933. Game management. Charles Scribner's Sons, New York.
Marion, W. K. 1974. Ecology of the Plain (diachalaca in the Lower Rio Grande Valley of Texas. Ph.D. thesis, Texas A&M Univ., College Station.
Merrill. J. C. 1878. Notes on the ornithoktgy of Southern Texas, being a list of birds observed in the vicinity of Fort Brown, Texas, from February 1876 to June 1878. Proc. U.S. Natl. Mus., Vol. 1, jip. 118 173.
Pethn(;ill, O. ,S., Jr. 1946. A laboratory and field manual of ornithology. Burgess Puhl. Co.. Minnea|)olis, Minnesota.
.Sutton, (L ,M. 1951. Mexican birds; first impressions. Univ. Oklahoma Press, Norman.
J arer, R. 1). 1963. (Iriteria of s(“x and age. Pp. 119-189, in Wildlife Investigational
Techniipies. (H. S. Moshy. ed. ) 2nd ed. The Wildlife Society, Washington, D. C.
AND 1. McT. (Iowan. 1963. Capturing and marking wild animals. Pp. 250-283,
in Wildlife Investigational Technicjues. (11. S. Moshy, ed.) 2nd ed. The W'ildlife .‘“'ociety, Washington, D. C.
CAESAR KLERERG RESEARCH l»ROGRAM L\ WILDLIFE ECOLOGY, DEPT. OF WILDLIFE AM) FISHERIES SCIENCES. TEXAS A&M UNIV., COLLEGE STATION 77845. PRESENT ADDRESS: WILDLIFE ECOLOGY LABORATORY, SCHOOL OF FOREST
RESOURCES AND CONSERVATION, UNIV. OF FLORIDA, GAINESVILLE 32611. ACCEPTED 1 OCT. 1675.
SOCIAL DOMINANCE IN WINTER ELOCKS OF CASSIN’S FINCH
Fred B. Samson
Reports of social dominance by females in avian winter flocks are few but have been described in the Bullfinch (Pyrrhula pyrrhula; Hinde 1955, 1956; Nicolai 1956) and the House Finch iCarpodacus mexicanus; Thomp- son 1960 ). I have noted this dominance in the Purple Finch i Carpodacus p. purpureus ) , and it is evident in this study of Cassin’s Finch ( Carpodacus cassinii) . The significance of female dominance in winter flocks is not known nor is the importance clearly evident for any pattern of avian social domi- nance during the winter (Watson and Moss 1970). The purpose of this study of winter flocks in Cassin’s Finch was to (1) assess patterns of social domi- nance, (2) suggest their possible ecological significance, and (3) describe displays involving agonistic or anti-predator behavior.
Cassin’s Finch is an irregular winter resident of the Cache Valley in north- ern Utah (K. L. Dixon, pers. comm.) where I studied flocks during the win- ters of 1972-73 and 1973-74. 1 found no flocks in the area in 1971-72 or 1974-75. Aside from fragmentary observations by those engaged in faunistic or winter surveys ( Orr 1968 and references cited therein ) , little is known of the winter behavior or biology of Cassin’s Finch.
METHODS
I observed the activity and social dominance of finches ahnost daily from January to April 1973 and an average of 2 days per week from November 1973 to February' 1974.
Five banding stations were established during the winter of 1972-73 at different sites within Cache Valley. All were at least 1 km apart with stations 1 to 4 in residential areas and station 5 at the mouth of Green Canyon. Cassin’s Finch visited only stations 2 and 3 during the second winter. I caught few finches in mist nets, but ca{)tured most in drop or walk-in traps baited with sunflower seeds and millet. Color of plumage was noted and wing lengths measured for all but 6 of 353 birds captured. Each bird was banded and I marked 131 with distinctive combinations of plastic color leg bands to permit later recog- nition without recapture.
Cassin’s Finch females and yearling males have a similar streaked gray-brown plumage, but all females during the breeding season exhibit an incubation patch and also can be distinguished by wing length (Samson 1976). Wing length measurements in 3 summer populations I studied in northern Utah and those obtained in this study are not signifi- cantly different either for older males or gray-brown birds ( Samson 1974) . A criterion based on wing length similar to that employed for summer populations is used in this study to separate females (wing lengths of 85.0 to 89.9 mm) and yearling males (wing lengths of 90.0 to 96.9 nnn). As discussed under head-forward display, feather arrange- ment also may be used to identify females during agonistic encounters.
I studied patterns of social dominance at or near banding stations. Finches concen-
57
58
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
|
Table 1 Location and Number of Cassin's Finches |
Banded |
AND Recaptured |
||||
|
Banding Station |
Nnmljer Banded |
Mean Per Day |
Number Recaptured at Banding Stations’^ |
|||
|
1 |
2 |
3 |
4 5 |
|||
|
1972-1973 |
||||||
|
1 |
131 |
9.4 |
26 |
26 |
33 |
17 2 |
|
2 |
42 |
14.0 |
12 |
11 |
11 |
9 |
|
3 |
64 |
5.3 |
8 |
20 |
19 |
10 |
|
4 |
51 |
17.0 |
14 |
18 |
23 |
11 |
|
1973-1974 |
||||||
|
2 |
38 |
4.8 |
29 |
26 |
||
|
3 |
21 |
2.6 |
18 |
19 |
^Aii iiiclividuiil bird may have J)een recaptured at more than 1 location.
Iraled tlu'ir activity near the hait and were not <d)scrvcd foraging elsewhere including adjacent mountain and vall(*y terrain which was regularly censused. Criteria of sub- ordination in agonistic encounters included the turning away or lateral body j)iesentation, avoidance, or fleeing of a finch relative to the approach of another individual. I also studied displays and social hi{*rarchies in 2 captive flocks (n = 6, n ir: 12) maintained in the summer of 1971. Linear social hierarchies construct(‘d from observed encounters among color-marked birds were noted in tbe 2 captive flocks but not in winter flocks and tberefore are not presented in this report. I'lie analysis of social dominance in early 1973 is subdivided by imuitb to consider the influence of possible ebanges in sex and age ratios on j)atterns in aggression. (.'bi-s(piare analyses of data were used to determine statistical significance.
Displays of individual (iassin's Finebes were rt'corded on 111 m of 8 mm c<dor movie film and 25 m of 35 mm black and white film during the second winter for later analysis.
SOCIAL DOMINANCE
l*()piil(iti()iis. - Of llie 288 fiiicdies handed in January to April of 1973 I lahle ll 80 were color-ltanded. Jdiroughout this winter unhanded finches were regularly observed and captured. Whether these birds represented im- migrants or uidtanded winter residents is not known nor is the total number of winter residents. Finches handed in mid-January were recaptured or ob- served in early A})ril, suggesting that birds remained for the winter. I caught 59 finches in early winter of 1973-74 (Table 1), and captured or observed few unhanded finches by mid- December 1973. Fifty-one of the 59 captured were color-handed, and these remained in the valley from late November 1973 into February 1974. Only one finch, a female banded in the first winter, was recaptured in the second.
Older males represented 21.9% (63 of 288) of finches banded in the winter
Samson • CASSIN'S FINCH
59
|
Social Dominance |
Table 2 IN Winter Flocks |
OF Cassin’s FinciF |
|
|
Subordinate Bird |
|||
|
Dominant Bird |
Female |
Older Male |
Yearling Male |
|
January 1973 |
|||
|
Female |
7 |
8 |
14 |
|
Older male |
2 |
9 |
13 |
|
Yearling male |
6 |
15 |
19 |
|
February 1973 |
|||
|
Female |
7 |
21 |
47 |
|
Older male |
9 |
13 |
39 |
|
Yearling male |
9 |
4 |
22 |
|
March 1973 |
|||
|
Female |
2 |
12 |
17 |
|
Older male |
16 |
8 |
|
|
Yearling male |
2 |
23 |
|
|
November 1973-February 1974 |
|||
|
Female |
31 |
134 |
264 |
|
Older male |
27 |
48 |
263 |
|
Yearling male |
21 |
112 |
140 |
1 Numbers refer to victories by group at left over individuals in the respective columns.
of 1972-73 and 54.2% (32 of 59) in 1973-74. Yearling males accounted for 48.6% ( 140 of 288 ) of birds banded in the first winter when finches were numerous in contrast to 18.6% (11 of 59) in the second. Females were out- numbered by all males 203:85 in 1972-73 and 43:16 in 1973-74. These sex ratios are similar to disparities favoring males reported by Samson ( 1976 ) in 3 breeding populations of Cassin’s Finch in northern Utah and to the proportion of males reported in over 15,000 Cassin’s Finches handed in North America from 1956 to 1973 (J. Sheppard, pers. comm.).
Patterns of social dominance. — Dominance-subordination in Cassin’s Finch winter flocks includes relationships between females, yearling males, and older males as well as between members of each group. Table 2 reflects the general dominance of females over both older and yearling males. The observed domi- nance by females over both male age classes is significantly different than expected in both winters (Table 3). Although not as successful in winning encounters as females, older males exceeded yearling males in proportion of encounters won in both winters (Table 2) and are dominant over the yearling male age class ( Table 3).
60
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
|
Comparison of Social |
Table 3 Dominance in Winter Flocks of Cassin’s |
Finch |
|
Dominance |
||
|
Kate |
Result |
|
|
January 1973 |
females > older males |
<.01 |
|
females > yearling males |
<.05 |
|
|
older males > yearling males |
<.01 |
|
|
Feliruary 1973 |
females > older males |
<.01 |
|
females > yearling males |
< .001 |
|
|
older males > yearling males |
< .001 |
|
|
March 1973 |
females > older males |
< .001 |
|
females > yearling males |
< .001 |
|
|
older males > yearling males |
< .05 |
|
|
Noveinhcr 1973-Fcl)ruary 1974 |
females > older males |
< .001 |
|
females > yearling males |
<.001 |
|
|
older males > yearling males |
< .001 |
1 Clii-S(iuare with dl = 1.
Heterosexual encounters most often occurred when a yearliiift; male ap- proached a feeding female or, rarely, when an older male attempted to sup- plant a female. In neither case were males regularly successful. Encounters of older males and females appeared to involve mistaken sex identification hy the male. Females were tolerant of other females, and 1 noted few inter- actions in either winter.
Many finches were captured at more than I location ( "Pahle 1). In both winters, observers at the different locations noted the temporal and spatial association of color-marked birds. Comi)arison of these records indicates that feeding flocks of Cassin’s Finch lack continuity in membership from day to day and from feeder to feeder on any specific day. Pairs did form in these flocks during late winter hut well after the establishment of patterns of social dominance, l^iir status could not have influenced social dominance exhibited hy unpaired females less than a year old over older and yearling males. Thus, the dominance of females as a grouj) appears independent of site, flock com- position, or mate status.
Winter disappearance. — The significance of female dominance in Cassin’s Finch may relate to improving their survival from breeding season to breed- ing season. In the winter of 1972-73, 64 of 85 females, 40 of 63 older males and 53 of 140 yearling males were recaptured at least 1 day following the initial handing. Significantly more females (P < .001 I were recaptured than
Samson • CASSIN’S FINCH
61
expected. Conversely, significantly fewer yearling males (P < .001) were recaptured than expected. Attempts to locate or observe marked individuals within Cache Valley or adjacent mountain terrain that were not among re- captures were unsuccessful, and I presumed they were dead or had moved from Cache Valley to seek another food source.
Fewer finches were winter residents in 1973-74 (Table 1 ) and few (n = 3j disappeared. The winter of 1973-74 was mild in comparison to 1972-73. Considering that the energy needs of a homeothermic animal increase as tem- perature decreases, both the milder winter conditions and fewer finches pres- ent to exploit available food resources may have contributed to the disap- pearance of few finches during the 1973-1974 winter.
DISPLAYS
Head- forward. — This display in Cassin’s Finch varied in intensity and, as in other finches ( Hinde 1955, 1956; Dilger 1960; Coutlee 1967), is divided into 2 categories, the low intensity head-forward display and the high intensity head-forward display. The closed beak is directed toward the opponent, the neck partially extended, legs slightly flexed, with the body tending toward a horizontal posture in the low intensity head-forward display (Fig. lA). If the aggressor is a female, the feathers of the forehead, breast, and hack are “shuffled” (Fig. IB) as in the House Finch (Thompson 1960). With females and yearling males nearly identical in plumage, this shuffling of feathers serves as a visual cue for sex identification in agonistic encounters. Rarely did females employ any other display to maintain their dominance or pre- ferential access to food or roost. Vocalizations did not accompany this or any other display.
Figure 1C depicts the high intensity head-forward display. The beak is usually but not always open, the head and body feathers are sleeked, and the long axis of the body is horizontal and in line with the opponent. If the opponent was above or below the attacker, the head was directed toward the opponent and the tail slightly raised. During the most intense head-forward displays, both wings were raised through rotation at the shoulder (Fig. ID). Although performed by females and older males, the high intensity head- forward display was especially evident in encounters between yearling males.
Combat. — I rarely noted combat (Fig. IE) between older males, among females, or in inter-sex encounters and did not observe it in the milder winter of 1973-74. Combat when evident usually occurred between yearling males. If a high intensity head-forward display was insufficient to dislodge an op- ponent, the attacker would proceed directly at the opponent with wings raised. If the opponent failed to yield, combat resulted. Combat did not result in noticeable body damage, and in most cases it was of short duration.
62
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
(E) comhat ; (F» suhmissive ; and ((0 anti-predator.
Often, the birds would fly up almost vertically continuing to engage in combat before one or both birds withdrew to separate perches. Beaks remained open and feet extended during the combat phase of these flights.
Submission. — When approached by an aggressor, suhmissive birds often
Samson • CASSIN'S FINCH
63
assumed an erect, stiff-legged posture leaning away from the attacker ( Fig. IF). If not directly approached hut in the presence of a dominant bird, sub- ordinate birds would flex their legs and assume a partially crouched posture similar to that described for other fringillids ( Hinde 1956, 1957; Thompson 1960; Coutlee 1967). This posture is similar to that observed when an avian predator was present (Fig. IG). Sharp-shinned Hawks (Accij)Lter striatus) , Cooper’s Hawks ( Accipiter cooperii ) , and Northern Shrikes ( Lanius excuhitor) were active and preyed on Cassin’s Finches near handing stations. Finches in this posture remained stationary moving only the upper throat until the predator departed. The legs were flexed so that the breast and abdomen nearly rested on the substrate.
Supplanting and avoidance. — As in the House Finch (Thompson 1960), I did not see special behavior by an attacking finch prior to supplanting a second bird. The direct or frontal presentation described for other Frin- gillidae (Hinde 1955, 1956) is apparent in Cassin’s Finch. In nearly all at- tempted supplants, the attacked bird flew l)efore the attacker landed. When the attacked bird did not flee, a lateral body presentation, a submissive pos- ture, or a slight fluffing of the feathers were considered indicators of avoid- ance. Aggressive chases among finches associated with supplanting were not observed either winter. Displacement activities (i.e., hill wiping, head scratch- ing, breast preening) were rarely observed in free-flying flocks hut were common in the 2 captive flocks.
DISCUSSION
Social dominance is not uncommon in avian winter flocks (Brian 1949, Sabine 1959, Dixon 1963, 1965; Kikkawa 1961, Zcihavi 1971). In these studies, males or males and their mates are reported dominant. In the House Finch (Thompson I960), Purple Finch and Cassin’s Finch, the members of this genus which breed in North America, females in winter flocks are either as or more dominant than males in agonistic encounters.
This social dominance in Cassin’s Finch is considered independent of loca- tion in contrast to the importance of site attachment in other species ( Brown 1963, Dixon 1963). It may he related to ( 1 ) their lack of annual fidelity to a winter area (Bailey and Niedrach 1965, Buckley 1973), (2) the lack of consistent flock organization as in certain other carduelines ( Newton 1972 ) , (3) the mobility of the species, or (4) the variable number of finches at a winter area which may range from none as in Cache Valley in 1971-72, 1974- 75 to over 5000 as reported in northern Colorado ( Chapin 1958 ) .
Other studies of finch populations during the winter (Fretwell 1969, Pul- liam and Enders 1971, Davis 1973) point out that food is important in deter- mining population levels and that intraspecific competition may influence
64
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
patterns of mortality. Newton ( 1964 ) provided evidence in the Bullfinch and Murton et al. ( 1966) in Wood Pigeon iColumba palumbus) that the avail- ability of winter food influences subsequent breeding population numbers. In Cassin’s Finch (Samson 1976) as in 2 other montane finches with sex ratios favoring males, the Black Rosy Finch {Lecousticte atrata, French 1959) and the Gray-crowned Rosy Finch ( L. tephrocotis, Johnson 1965), the num- ber of females is considered the limiting resource for reproductive effort. The significance of female dominance in Cassin’s Finch appears to involve the pro- tection of this limiting resource during the non-breeding season. Survival of females is enhanced by preferential access to food and roost sites in winter, thus allowing for maximization of reproductive effort during the subsequent breeding season. Considering that Cassin’s Finch, lacking a strong fidelity to a wintering area or breeding area, must colonize new wintering and breed- ing areas annually, a reproductive strategy to maximize reproductive effort may represent an important correlate to their nomadic lifestyle and enhance the efficient use of an unpredictable environment ( i.e., food and weather). dTiese habitat and species correlates all pertain to an r-strategy ( Pianka 1970). Opportunism and reproductive strategy in North American birds have not, however, been intensively studied (Cody 1972).
J he displays used l)y Cassin’s Finch in agonistic encounters are generally homologous to those of the House Finch and to other fringillids ( Hinde 1955, 1956; Coutlee 1967). Cassin’s Finch does differ from many fringillids in that vocalizations did not accompany displays. This was particularly evident in intersi)ecific encounters between the Cassin’s Finch and the House Finch, the latter regularly using vocalizations in association with certain intense agonistic displays.
In nearly all phases of its annual cycle, Cassin’s Finch tends to flock. The flocks are characterized by an al)sence of agonistic encounters except in winter and in those of yearling males which remain at high altitudes in late summer after other Cassin’s Finches have departed. Except among yearling males, the lack of intense agonistic encounters observed in this study may contribute to the flocking tendency. Aggressive behavior did increase at a food source as in the House Finch (Thompson I960), but this increase was not as substantial as that observed in early 1973 when weather conditions were severe and finches numerous. Nor, was it as intense as in yearling male flocks in late summer (Samson 1976).
Females and yearling male Cassin’s Finches are well camouflaged in their striped gray-brown plumage when roosting on woody branches or foraging under a forest or shrub canopy. This coloration combined with the motion- less anti-predator posture may enhance their survival from breeding season to l)reeding season. However, the explanation for the imbalance in the sex
Samson • CASSIN’S FINCH
65
ratios, subadult male plumage, and possible hormonal factors influencing fe- male dominance in Cassin’s Finch remains to be resolved.
SUMMARY
Female Cassin’s Finches were determined socially dominant over older and yearling males in flocks during 2 winters. Few females disappeared either winter in contrast to males. With number of females limiting for breeding effort, the dominance of females in winter is interpreted as a behavioral modification to maximize reproductive effort. This species’ trait and the need to semiannually colonize a new and often unpredictable environ- ment are correlates of an r-strategy. Displays in agonistic encounters are considered homologous to other fringillids. Reasons for the observed disparities in sex ratio or hor- monal factors influencing female dominance are not known.
ACKNOWLEDGMENTS
I thank A. W. Stokes, M. H. Ralph, and, in particular, F. L. Knopf for their assistance in the field. K. L. Dixon’s assistance throughout the study and his comments as well as those of C. F. Thompson on an earlier draft of the manuscript are appreciated. S. Samson provided valuable help in preparing the manuscript. J. Sidelinger prepared the drawings.
I Financial support came from Sigma Xi and the Chapman Fund, American Museum of Natural History, New York.
LITERATURE CITED
Bailey, A. M. and R. J. Niedracii. 1965. Birds of Colorado, Vol. 2. Denver Mus. of Nat. History, Denver.
Brian, A. D. 1949. Dominance in the Great Tit Pants major. Scott. Nat. 61:144-155. Brown, J. L. 1963. Aggressiveness, dominance and social organization in the Steller Jay. Condor 65:460-484.
Buckley, P. A. 1973. The changing seasons. Am. Birds 27:578-585.
Chapin, J. 1958. The 1957 Christmas counts in Colorado. Colorado Bird Notes 5:29-31. Cody, M. L. 1972. Ecological aspects of reproduction. Pp. 462-512, in Avian biology, Vol. 2. (D. S. Farner and J. R. King, eds.). Academic Press, N. Y.
Coutlee, E. L. 1967. Agonistic behavior in the American Goldfinch. Wilson Bull. 79: 89-109.
' Davis, J. 1973. Habitat preferences and comjietition of wintering Juncos and Golden- i crowned Sparrows. Ecology 54:174-180.
Dilger, W. C. 1960. Agonistic and social behavior of captive Redpolls. Wilson Bull. 72:115-132.
Dixon, K. L. 1963. Some aspects of social organization in the Carolina Chickadees. Proc. 13th Int. Ornithol. Congr. : 240-258.
. 1965. Dominance-subordination relationships in Mountain Chickadees. Condor
67:291-299.
French, N. R. 1959. Life history of the Black Rosy Finch. Auk 76:159-180. Fretwell, S. 1969. Dominance behavior and winter habitat distribution in juncos (Junco hyemalis) . Bird-Banding 40:1-25.
Hinde, R. a. 1955, 1956. A comparative study of the courtship of certain finches (Fringillidae) . Ibis 97:706-745; 98:1-23.
: Johnson, R. E. 1965. Reproductive activities of Rosy Finches, with special reference
to Montana. Auk 82:190-205.
66
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
Kikkawa, J. 1961. Social behavior of the White-eye Zosterops lateralis in winter flocks. Ibis 103:428-442.
Murton, R. K., a. J. Isaacson, and N. J. Westwood. 1966. The relationships between Wood-pigeons and their clover food supply and the mechanism of population control. J. Appl. Ecol. 3:55-96.
Newton, I. 1964. Bud-eating by Bullfinches in relation to the natural food supply. J. Appl. Ecol. 1:265-279.
. 1972. Finches. Taplinger Publ. Co., N. Y.
Nicolai, J. 1956. On the biology and ethology of the Bullfinch iPyrrhula pyrrhula L.). Z. Tierpsychol. 13:93-132.
Ohh, R. T. 1968. Cassin's Finch Carpodacus cassinii Baird. Pp. 280-289, in Life his- tories of North American cardinals, grosbeaks, buntings, towhees, finches, sparrows, and allies (A. C. Bent and collaborators), U.S. Natl. Mus. Bull. 237.
PiANKA, E. R. 1970. On r- and A--selection. Am. Nat. 104:592-597.
Pulliam, H. R. and F. Enders. 1971. The feeding ecology of five sympatric finch spe- cies. Ecology 52:. 557-566.
Sarine, W. S. 1959. The winter society of the Oregon Junco: intolerance, dominance, and the pecking order. Condor 61:110-135.
Samson, F. B. 1974. On determining sex and age in the Cassin's Finch. Western Bird Bander 49:4-7.
1976. Territory, breeding density and fall departure in Cassin’s Finch. Auk 93:477-497.
d'lioMPSON, W. L. 1960. Agonistic behavior in the House Finch. Part 2: Factors in aggressiveness and sociality. Condor 62:378-402.
Watson, A. and R. Moss. 1970. Dominance, spacing behavior and aggression in rela- tion to poj)ulation limitation in vertebrates. Pp. 167-220, in Animal populations in relation to tlu*ir food resources (A, Watson, ed.l. Blackwell Scientific Publ., Oxford.
Zaiiavi, a. 1971. Tin* s<(cial behaviour of the White Wagtail Motacilla alha alba win- tering in Israel. Ibis 113:203-211.
DKI’T. OF HIOLOGV, UTAH STATE UMV., LOGAN, UTAH (IL322. ( PItESENT ADDRESS:
COOPERATIVE WILDLIFE RESEAltGII UNIT, STEPHENS HALL, UNTV. OF MIS- SOURI, GOLUMRIA 6.5201.) AGGEPIED 20 .JAN. 1976.
INTER-BROOD MOVEMENTS OF JUVENII.E SPRUCE GROUSE
f)ANIEL M. KePPIE
Juvenile tetraonids form broods during most of their first summer of life. Undoubtedly this contact between parent and offspring has survival value for the chicks, particularly during development of thermoregulation and per- haps for acquisition of learned behavior. Presumably, a brood consists of a single family, yet a survey of 26 theses and published reports pertaining to brood size and behavior of tetraonids revealed 10 in which the author sus- pected broods contained chicks from different hens (Lehmann 1941, Wing et al. 1944, Bump et al. 1947:293, Patterson 1952:135, Bendell 1955, Chambers and Sharp 1958, Dalke et al. 1963, Bendell and Elliott 1967, Braun and Rogers 1971, Harju 1974). But the evidence is based only on observations of juveniles of different sizes or estimated ages or, supposedly, an excess number of chicks. Inter-brood movements among tetraonids apparently do not occur on the massive scale reported for aquatic birds ( Beard 1964, Gorman and Milne 1972). There is little known about the freciuency of occurrence and circumstances surrounding inter-brood movements of grouse.
Herein I document inter-brood movements of juvenile Spruce Grouse (Canachites canadensis)^ calculate their frequency of occurrence, and briefly (luestion the function of broods remaining as individual units over a long period of time.
METHODS
Data were gathered from marked l)irds incidental to a population study at Gorge Creek (GC) and Blue Rock Creek (BRC), 27-32 km west of Turner Valley, Alberta from 1970 through 1973. Grouse were located hy repeatedly searching the study areas with trained pointing dogs. All hens with broods known to he on the study areas were captured and marked. Numbers of juveniles and their survival were determined from counts of brood size and records of marked individuals. Young chicks were marked with numbered wing tags (size no. 1, National Band and Tag, Newport, Ky. ) ; leg hands were used after about 40 days of age. Juvenile age was determined hy growth of primaries ( McCourt and Keppie 1975).
RESULTS
The efficiency of tagging grouse is shown in Table 1. In 3 of the 4 summers, over 50% of the maximum number of juveniles seen were marked hy 14 days of age and about 80% hy 42 days of age. All juveniles that survived until the end of the brood period were marked hy that time.
Broods were seen on 428 occasions in 4 years; in only 8 instances (2%)
67
68
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
|
Table Percentages of Juvenile Spruce 28, AND 42 Days of |
1 Grouse Marked Age, 1970-73 |
BY 7, 14, |
||||
|
Year and Area |
||||||
|
1970 |
1971 |
1972 |
1973 |
|||
|
GC |
BRC |
GC |
GC |
GC |
||
|
Age of bird |
(66)* |
(30) |
(103) |
(88) |
(89) |
|
|
( days ) |
(53)** |
(24) |
(97) |
(84) |
(77) |
|
|
7 |
6 |
0 |
35 |
43 |
39 |
|
|
14 |
9 |
3 |
56 |
58 |
54 |
|
|
28 |
32 |
13 |
70 |
68 |
71 |
|
|
42 |
52 |
23 |
84 |
81 |
79 |
* Maximum number of different juveniles sijihted. ** Number juveniles marked in brood period.
were 2 found within 50 m of each other (Table 2), but brood ranges were not mutually exclusive. Nineteen % of the occupied habitat (all years) was included in overlap, i.e., within home ranges of different hens. Some habitat was within the home range of up to 4 families. In one instance (GC, 1971 j, 6 broods were present on a 108 ha plot, and although 37% of the area was used by 2 or more broods only 2% (1/44) of the sightings were of 2 broods together. In all years, overlap of brood ranges probably was greater than recorded because sightings of broods were limited.
Among the 8 occasions that 2 broods were together, 33 juveniles were already tagged and 32 of these were seen with the same hen at a later date. The remaining juvenile joined the second hen when the 2 broods came together. In one sighting of 2 broods, both hens simultaneously called to their chicks from adjacent trees yet the 4 marked juveniles were seen later with the “correct” adult.
riiere were 12 instances of 11 marked juveniles (7 females, 4 males) moving from a total of 7 broods. Four of the 7 hens were known or assumed to have died. Ihree juveniles moved from 3 other hens that remained alive, hut one of these juveniles was i)reviously orphaned. In all cases in which the hen remained alive (3) only one juvenile moved to a new brood; the only instances (2) of siblings moving together to a new family occurred when the original hen died. All 11 juveniles were at least 11 days old when mixing occurred and all joined broods that occupied overlapping or adjacent home ranges. 4Tn of these juveniles (91%) survived until at least the end of summer. Survival beyond summer was not determined because many juveniles dispersed in autumn.
Seven hens were known or assumed to have died while with juveniles; 6
Keppie • INTER-UROOI) MOVEMENTS OE (;R0USE
69
Table 2
Summary of Brood Sightings and Inter-brood Movements of Juvenile Spruce Grouse, 1970-73
Year and Area
1970
|
GC |
BBC |
1971 GC |
1972 GC |
1973 GC |
All Yrs |
|
|
Size of area (km^) |
5.2 |
1.6 |
6.8 |
6.8 |
6.8 |
- |
|
No. broods^ |
15 |
6 |
20 |
20 |
18 |
79 |
|
broods/km^ |
2.9 |
3.8 |
3.9 |
3.9 |
3.5 |
- |
|
No. contacts with broods |
57 |
28 |
118 |
121 |
104 |
428 |
|
No. sightings of 2 broods/contact |
1 |
2 |
1 |
2 |
2 |
8 |
|
% of total |
1.8 |
7.1 |
0.8 |
1.7 |
1.9 |
1.9 |
|
No. juveniles alive after |
||||||
|
about 2 weeks of age |
56 |
29 |
69 |
50 |
62 |
266 |
|
No. marked juvs. known |
||||||
|
to change broods |
1 |
1 |
4 |
1 |
4 |
11 |
|
% of total |
1.8 |
3.5 |
5.8 |
2.0 |
6.5 |
4.1 |
1 Broods are excluded if aU chicks were lost early.
deaths were at least 11 days after hatch (11-22, 17-22, 17-30, 40, 40-43, and 54 days). In this sample, the probable number of chicks alive when the hens died was 27, of which 20 (74%) were still alive at the end of summer (2
j others died from handling) . By contrast, a hen died at 4-9 days post-hatch and her 3 chicks were not seen again. This brood was rather isolated and if the juveniles did not die at the same time as the hen they may have had difficulty locating another family.
Although data are limited and not clear-cut, the “need” for orphaned
j juveniles to seek out a new brood may have varied with age. In 3 broods the
I hen died before 30 days post-hatch; juveniles in 2 of these joined new families within 9 days, and juveniles in the third brood were captured with a new hen 28 days after the death of the hen. A brood count suggested juveniles in this latter brood were present in a new brood 10 days after the
' original hen died. Three broods were orphaned after 40 days of age and
II the juveniles were seen later as intact units without a hen. Juveniles of one of these latter broods were without a new hen for at least 26 days, and only one
I of the 7 chicks then joined a new family; juveniles in the other 2 broods
I remained together for 8 and 11 days until they dispersed.
The frequency of brood interchange was calculated from the number of
II juveniles remaining after about 2 weeks of age (Table 2) because this probably excludes most of the high losses from natural mortality (Zwickel
I
70
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
and Bendell 1967). Four % of the juveniles surviving beyond about 2 weeks of age were known to join other broods. At GC, slightly greater mixing occurred in 1971 and 1973 and resulted from several siblings, rather than one chick per brood, moving to other families.
DISCUSSION
Although brood ranges of Spruce Grouse were not mutually exclusive, there were few documented exchanges of juveniles between broods. Proportions of juveniles recorded changing broods probably were overestimates for the cohort hatched unless considerable mixing occurred before chicks were marked.
I he freciuency of 2 broods coming together likely was greater than recorded, hut when considered on a temporal basis such gatherings probably still constituted a small iiroportion of the brood period. Call notes seemingly function to maintain contact between hen and chicks ( Zwickel 1967, this study), and individuality of call notes might facilitate proper reorganization when broods come together. Although there were few records of 2 broods together, the observation that juveniles reunited with their respective parent, coupled with individuality of sound, generates the <}uestion of whether survival of a juvenile is enhanced by staying with its respective mother. At least short term survival was good for juvenile Spruce Grouse that changed broods; survival also was good for juveniles that were orphaned. Survival of orphaned juveniles might he age related, recpiiring the full development of thermo- regulation, and for young chicks (<2 weeks old) the proximity of another brood might he critical to survival.
Several authors have speculated on causes of inter-hrood movements of grouse, such as the death of a hen (Bump et al. 1947:293), a loose feeding formation and lack of an efficient rallying call (Lehmann 1941), and a concentration of broods (Wing et al. 1944, Bendell 1955). Bendell (1955) further suggested weather as the ultimate cause, by its influence on plant growth and distribution of preferred feeding sites. Death of hen Spruce (house seemed to he a cause for juveniles switching broods, hut perhaps only when juveniles were young. There was no evidence of a weak cohesion among family members, concentration of broods, nor preferred or localized feeding sites. I do now know whether densities of broods in this study were high for Spruce Grouse; they were generally as high or higher than densities recorded by others (Ellison 1974, McCourt 1969). It is open to (juestion whether higher densities might reduce the effectiveness of calling for maintaining brood organization, resulting in greater exchange of juveniles. There was no effect of movements between broods in summer on mean brood size. Many juveniles temporarily join other families while dispersing in autumn
Keppie • INTEK-HKOOl) MOVEMENTS OE (;H()IJSE
71
j (Keppie, unpubl. data) and biases on counts of brood size would be greatest i at that time.
Whether a specific mechanism or simple chance accounts for separation of I grouse broods ( Bendell and Elliott 1967, Zwickel 1973, Godfrey 1975; this study) is unknown. Dispersion of broods may result from other factors oper- . ating earlier during courtship and nesting. Perhaps brood dispersion enhances survival of the chicks, but present data on survival until autumn for juveniles I switching broods do not support this idea. If juveniles that move to a different
brood survive, and if juveniles of a certain age can live without a hen, we
should focus attention on the purpose of the dispersion pattern and why hen and chicks maintain contact longer than seems necessary.
SUMMARY
Inter-brood movements of juvenile Spruee (irouse were reeorded in Alberta from 1970 j through 1973. Although brood ranges were not mutually exclusive, broods generally I maintained their original constituency. Only 4% of the marked juveniles changed j broods; they moved from 7 broods and in 4 cases the hen had died. All juveniles that
' moved were at least 11 days old and all joined a family in the immediate vicinity.
; Juveniles that changed hroods or which were orphaned survived well until autumn.
Although data are limited on the fate of juveniles that mix or which are orphaned, the
I question arises as to why hroods exist as individual units for perhaps longer than
necessary to ensure survival of the chicks.
i ACKNOWLEDGMENTS
Financial support was provided by the National Kesearch (Council of Canada and the National Wildlife Federation; I). A. Boag, Univ. of Alberta, provided funds for logistical support. I acknowledge assistance in the field by students in the Department of Zoology, Univ. of Alberta: W. E. Etherington, A. Garbutt, M. Henderson, R. Salter, K. Smith, and || D. Thompson. I am thankful for advice on the manuscript from F. C. Zwickel and J.
I, Kristensen, Univ. of Alberta, C. E. Braun, Colorado Division of Wildlife, and T. Dilworth,
1 Univ. of New Brunswick. The University of New Brunswick has supported the costs of
I publication.
I LITERATURE CITED
Beaki), E. B. 1964. Duck brood behavior at the Seney National Wildlife Refuge. J. j. Wildl. Manage. 28:492-521.
Bendell, J. F. 1955. Age, breeding behavior and migration of Sooty Grouse, Dendrag- apus obscurus fuliginosus (Ridgeway). Trans. N. Am. Wildl. Conf. 20:367-381.
AND P. W. Elliott. 1967. Behaviour and the regulation of numbers in Blue
I Grouse. Can. Wildl. Serv. Rep. Ser. 4.
i Braun, C. E. and G. E. Rogers. 1971. The White-tailed Ptarmigan in Colorado, j Colorado Div. Game, Fish and Parks, Tech. Publ. 27.
1 Bump, G., R. W. Darrow, F. C. Edminster, and W. F. Crissey. 1947. The Ruffed
■ Grouse — life history, propagation, management. New York State Conserv. Dept.
72
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
CiiAMBEits, R. E. AND W. M. Sharp. 1958. Movement and dispersal within a population of Ruffed Grouse. J. Wildl. Manage. 22:231-239.
Dalke, P. D., D. B. Pyrah, U. C. Stanton, J. E. Crawford, and E. F. Schlatterer. 1963. Ecology, productivity, and management of Sage Grouse in Idaho. J. Wildl. Manage. 27:811-841.
Ellison, L. N. 1974. Population characteristics of Alaska Spruce Grouse. J. Wildl. Manage. 38:383-395.
Godfrey, G. A. 1975. Home range characteristics of Ruffed Grouse broods in Minnesota. J. Wildl. Manage. 39:287-298.
Gorman, M. L. and H. Milne. 1972. Creche liehavior in the Common Eider, Somateria m. mollissima L. Ornis Scand. 3:21-25.
Harju, H. j. 1974. An analysis of some aspects of the ecology of Dusky Grouse. Ph.D. Thesis, Univ. Wyoming, Laramie.
Lehmann, V. W. 1941. Attwater’s Prairie Chicken. Its life history and management. North Am. Fauna 57.
McCourt, K. H. 1969. Dispersion and dispersal of female and juvenile Franklin’s Grouse in southwestern Alberta. M.S. Thesis, Univ. Alberta, Edmonton.
AND 1). M. Keppie. 1975. Age determination of juvenile Spruce Grouse. J. Wildl.
Manage. 39:790-794.
I^ATTERSON, R. L. 1952. The Sage Grouse in Wyoming. Sage Books, Inc., Denver. Wing, L., J. Beer, and W. Tidyman. 1944. Brood habits and growth of Blue Grouse. Auk 61:426-440.
ZwiCKEL, F. C. 1967. Early behavior in young Blue Grouse. Murrelet 48:2-7.
. 1973. Dispersion of female Blue Grouse during the brood season. Condor 75:
114-119.
AND .1. F. Bendell. 1967. Early mortality and the regulation of numbers in Blue
Grouse. Can. J. Zool. 45:817-851.
DEPT. OF ZOOLOGY, UNIV. OF ALDEKTA, EDMONTON (PRESENT ADDRESS, DEPTS. OF FOREST RESOURCES AND BIOLOGY, UNIV. OF NEW BRUNSWICK, FREDERIC- TON, NEW BRUNSWICK, CANADA EBB 5A3). ACCEPTED 7 APR. 1976.
BREEDING BIOLOGY OE YEAR-OLD AND OLDER FEMALE RED-WINGED AND YELLOW-HEADED BLACKBIRDS
Richard J). Crawford
Age of male Recl-wingecl (Agelaius phoeniceus) and Yellow-headed black- birds {Xanthocephalus xanthocephalus) is known to influence their breeding behavior (Orians 1961, Willson 1966), but little effort has been devoted to the comparative breeding biology between age classes of females. Both species are polygynous (Verner and Willson 1969) ; how age might affect the females within this system, however, is unknown. The objective of this study was to compare data on selected parameters of breeding between yearling and older adult females of both species.
Field work was conducted from 1972-1974 on Dewey’s Pasture and Dan Green Slough, 2 glacial marshes in northwestern Iowa that have been described by Bennett (1938) .
METHODS
Red-wings were aged by using the methods presented hy Nero (1954, 1961) and Meanley and Bond (1970) ; yearling females have a pink or salmon epaulet and light pink chin and face, while older females show a more crimson epaulet and dark pink chin and face. To verify this aging technique, 1 initiated a handing program in 1972. Eighteen females were recaptured in years after their handing, 7 of which were yearlings. Both yearlings and older adults showed patterns similar to those described.
No similar aging technique exists for Yellow-heads, hut Bent (1958:112) described the first-year female as “much like the adults, hut colors are more veiled.” By examining 21 returning marked females (11 yearlings and 10 older adults), I was able to establish that yearlings have lighter breasts, throats, and facial regions than older adults.
A major advantage of the aging techniques for both species is that an observer can readily distinguish ages in the field.
Observations on the nesting activities of both species began in late May of each year. Most females were captured, handed, and classed as either year-old or older. The few females not trapped were aged in the field hy the methods described. After incubation had begun, nests were checked only every 2 or 3 days to minimize disturbance; the nestlings used for growth rate studies, however, were checked daily.
The date the first egg was laid was used as an indicator of nesting chronology. If not known precisely, this date was estimated, considering 12 days to he an average incubation period for both species (Nero 1961, Willson 1966). Only first nests were used for analysis. Statistical comparisons were made with Student’s t-test (Steel and Torrie 1960).
RESULTS
Nesting chronology, clutch size, and egg size. — Yearling Red-wing and Yellow-head females began nesting an average of 15 and 16 days later, respec-
73
74
THE WILSON BULLETIN • Vol. 89, No. 1, March 1977
|
Dates of |
Table 1 Nest Initiation (1972-1974) |
||
|
N |
Mean ±; 2 S.E. (days) |
Range |
|
|
KW yearling’ |
67 |
8 June ± 5.2 |
25 May-18 June |
|
KW older adult |
41 |
25 May ± 5.7 |
19 May- 5 June |
|
YII yearling |
71 |
7 June ± 4.9 |
22 May-15 June |
|
Yll older adult |
49 |
23 May ± 5.4 |
18 May-29 May |
|
1 RW = Red-winged Rlackhird, YH |
= Yellow-headed Blackbird. |
lively, than older adults (Table 1). ITie yearlings also showed a greater range in nest initiation dates. The differences between nest initiation dates of yearlings and older adults were highly significant for both species ( P < 0.01).
Means and frecjuencies of clutch sizes observed are given in Table 2. Yearling females of both species had significantly smaller clutches than older females (P<0.01 I. ddie mean values are similar to those of other studies. 1 he average clutch size was 3.1 for 501 Yellow-head nests in Iowa ( Ammann 103o); 3.7 for I IB nests in I tah ( Fautin 1041); and 3.6 for 371 nests in Washington (Willson 1066). For the Ked-wing, average clutch sizes reported were 3.5 for 026 nests in New York (Case and Hewitt 1063) ; 3.4 for 243 nests in Oklahoma ( Goddard and Board 1067); and 4.2 for 13 nests in Missouri ( Craw ford 1070 ) .
Yearling females of both species laid significantly shorter eggs than older females (P<0.01) (Table 3). Fgg width did not differ significantly for either species (P > 0.05). The mean values for both length and width are similar to those found in other studies (Bendire 1B05. Reed 1065).
|
Table 2 |
||||||
|
Ui.r'Kai |
Size and |
Fl.EDCINt; Sl'CCESS |
( 1972-1974) |
|||
|
Clutch Size |
Mean Fledged Young Per Nest |
|||||
|
2 |
3 |
4 |
.5 |
Mean |
||
|
B\\' yt'arling |
4’ |
37 |
34 |
3.40 |
0.73 (71)- |
|
|
HW older adult |
1 |
40 |
6 |
4.11 |
1.63 (40) |
|
|
HW all ages |
4 |
.38 |
74 |
6 |
3.67 |
1.05(111) |
|
’i H yearling |
8 |
49 |
31 |
3.30 |
0.87 (83) |
|
|
^ 11 older adult |
51 |
6 |
4.11 |
1.81 (52) |
||
|
H all ages |
8 |
49 |
82 |
6 |
3.62 |
1.30(135) |
1 X uni her of clutches.
~ Sample size in parentheses.
Crawford • HLACKUIRI) HHEE1)IN(; mOLOGY 75
|
Table 3 Egg Sizes (1972-1974) |
|||
|
Mean ( mm ) |
|||
|
N |
Length |
Width |
|
|
RW yearling |
165 |
22.1 |
17.0 |
|
RW older adult |
145 |
26.8 |
18.2 |
|
YH yearling |
181 |
23.2 |
17.3 |
|
YH older adult |
157 |
27.1 |
18.9 |
Nestling; growth and fledging success. — At hatching;, nestling;s of yearling I females averaged only slightly smaller than those of older adults; weights at 10
3 days of age, however, were significantly lower for nestlings reared by
^ yearling females (P <0.()1) (Table 4). Male nestlings of both species have
I been reported to grow faster and attain greater weights at fledging than females
• of the same age ( Ammann 1938, Williams 1940, Holcomb and Twiest 1971).
! I assumed in this study that the sex ratio was constant throughout the nestling i period and that differences in sex-specific weights would have no net effect.
Fledging success is given in Table 2. Yearling females of both species I fledged significantly fewer young than did older females (P<0.01). The fledging successes reported in this study are similar to those reported else- where (Wood 1938, Willson 1966, Goddard and Board 1967).
Pairing status of age classes. — Data on pairing status and its relationship to age were collected on 30 Red-wing and 20 Yellow-head territories during
1'able 4
|
Growth |
IN Weight (g) of |
Nestlings (1972-1974) |
||
|
Age (days) |
Red-wing |
Yellow-head |
||
|
yearling |
older adult |
yearling |
older adidt |
|
|
1 |
3.6 (65)' |
4.0 (44) |
3.9 (66) |
4.1 (49) |
|
2 |
5.8 (48) |
6.1 (37) |
6.8 (50) |
7.0 (40) |
|
3 |
8.8 (47) |
9.5 (30) |
10.4 (49) |
10.8 (36) |
|
4 |
12.7 (39) |
13.7 (27) |
16.5 (43) |
16.8 (35) |
|
5 |
16.6 (36) |
18.9 (23) |
22.0 (40) |
22.5 (29) |
|
6 |
21.1 (28) |
24.2 (22) |
28.7 (33) |
29.9 (24) |
|
7 |
25.9 (19) |
27.3 (20) |
33.1 (24) |
35.8 (19) |
|
8 |
28.3 (15) |
31.6 (16) |
37.3 (16) |
40.7 (17) |
|
9 |
30.4 (11) |
34.3 (13) |
40.5 (13) |
45.1 (15) |
|
10 |
32.1 (10) |
37.4 (12) |
43.7 (10) |
49.3 (11) |
1 Sample size in parentheses.
76
THE WILSON BULLETIN • VoL 89, No. 1, March 1977
Table 5
|
Mean Clutch |
Size anu Fledging Success in Status (1973-1974) |
Relation |
TO Pairing |
|||
|
Primary Female |
Secondary Female |
|||||
|
N |
CSi |
FSi |
N |
CS |
FS |
|
|
KW yearling |
6 |
4.0 |
1.2 |
29 |
2.9 |
0.6 |
|
KW older adult |
24 |
4.2 |
1.8 |
11 |
4.1 |
1.3 |
|
YH yearling |
7 |
4.0 |
2.0 |
17 |
2.8 |
0.8 |
|
YH older adult |
13 |
4.1 |
2.0 |
4 |
4.0 |
1.2 |
1 (]S = clutch size, FS - fledging success.
1973-74. Herein, I use the terminology of Martin (1974) ; the first female to nest in a male’s territory is referred to as the primary female, and all those nesting suhsetiuently are termed secondary females.
Pahle 5 i)resents data for females where pairing status was determined. For the Red- wing, most primary females were older adults. Some older adults were secondary females, hut in all such instances, the primary female was also an older adult. Only 6 yearlings were primary females, 4 of which mated monogamously, and the other 2 mated i)olygynously where the secondary females were also yearlings. In no instance was a yearling female the primary mate and an older female secondary within the same territory.
A similar situation existed for the Yellowdiead (Table 5). All yearling primary females mated monogamously, and all older adult secondary females were secondary only to other older adults.
Further evidence to suggest that age is an important factor influencing pairing status was gained hy examining data from returning females of known age rkahle 6). In most instances, females were secondary as yearlings and primary when 2 years old; 2 females, however, were secondary both as yearlings and as 2-year-old birds, and 1 female was primary when a yearling as well as when she was 2 years old.
Po determine if differences in breeding biology existed in relation to pairing status, data were compared between yearling and older females of both species ( Pahle 5). Older adult primary females did not show significantly larger clutches than older adult secondary females (P > 0.05), hut primary yearling females of both species laid significantly larger clutches than did secondary yearling females (P<0.01). Trends similar to these also were found in the clutch sizes of known-age females (Tal)le 6). Yearling females fledged signifi- cantly ( P < 0.01 I fewer young than did older adults for all pairing situations, except for primary Yellow-heads (Table 5). Yearling and older adult primary females of both species fledged significantly more young than did secondary
Crawford • BLACKBIRD BREEDING BIOLOGY
77
|
Pairing Status |
AND Clutch Size |
Table 6 OF Returning Known-age |
Females |
(1973-1974)^ |
|
Species |
Female No. |
Yearling |
Age |
2 years old |
|
Red-wing |
DP74 |
IP (3) |
P (4) |
|
|
DP97 |
IP (3) |
P (4) |
||
|
DP 189 |
IP (2) |
IP (3) |
||
|
DP191 |
IP (4) |
P (4) |
||
|
Yellow-head |
I)P67 |
IP (3) |
P (5) |
|
|
DP96 |
IP (3) |
P (4) |
||
|
DP157 |
IP (3) |
IP (3) |
||
|
DP181 |
P (4) |
P (5) |
||
|
DP192 |
IP (3) |
P (4) |
||
|
DP197 |
IP (3) |
P (4) |
1 1° = primary female, 11° = secondary female, number in parentheses is clutch size.
1 females (P<0.01). Egg size within age groups was not influenced by I pairing status. Sample sizes were inadetjuate to analyze differences in nesting J chronology or fledgling weight between primary and secondary females.
DISCUSSION
i
' Lighter-colored females have been noted several times in breeding popula- l[ tions of both species (Nero 1954, Bent 1958, Strosnider I960),