Female Pairing: a Reproductive Strategy for Herring Gulls? ’
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SHORT COMMUNICATIONS 933 it immediately (Howell, pers. comm.). The occurrence LITERATURE CITED of this speciesin Costa Rica is to be expected,as it has been expandingits range in Nicaragua in recent years; AMERICANORNITHOLOGISTS UNION.’ 1983. Check- it is now known they are south at least to Rivas, just list of North American birds. 6th ed. American north of the Costa Rican boundary (J. C. Martinez, Ornithologists’ Union, Washington, DC. pers. comm.). HILTY, S. L., AND W. L. BROWN. 1986. A guide to the birds of Colombia. Princeton Univ. Press, LINCOLNS’ SPARROW Princeton, NJ. (MELOSPIZA LINCOLNII) MILLER,W. D., AND L. GRISCOM. 1925. Descriptions of new birds from Nicaragua.Am. Mus. Nat. Hist. I collected an immature female of this species in a Novitates, no. 159. brushy field 0.5 km northeastof Ciudad Universitaria, RIDGELY,R. S. 1981. A guide to the birds of Panama. Prov. San Jose, on 16 November 1985. The specimen 2nd ed. Princeton Univ. Press, Princeton, NJ. (UCR 3238) in fresh plumage with very light fat, is RIDGWAY.R. 1914. The birds of North and Middle the first for Costa Rica following several sight reports America. Part 6. Bull. U.S. Natl. Mus. 50. (Tramer 1979, Stiles and Smith 1980). In addition, G. SLUD,P. 1964. The birds of Costa Rica: distribution Barrantes, A. Pereira, and I banded and released an and ecology. Bull. Am. Mus. Nat. Hist. 128. adult Lincoln’s Sparrow at the same site on 25 No- STILES,F. G., AND A. F. SKUTCH. In press. A guide vember 1985. to the birds of Costa Rica. Cornell Univ. Press, I wish to express my gratitude to the various ob- Ithaca, NY. servers mentioned above for their help and informa- STILES,F. G., AND S. M. SMITH. 1980. Notes on bird tion. In addition, I thank the Organization for Tropical distribution in Costa Rica. Brenesia 17:137-l 56. Studies-Proyecto Zona Norte for logistical support TRAMER,E. J. 1979. First sight records of Lincoln’s during the collection of the Ocellated Poorwill, and Sparrowfor Costa Rica. Wilson Bull. 9 1:469-470. the Vicerrectoria de Investigation, Universidad de WETMORE,A. 1968. The birds of the republic of Pan- Costa Rica, for financial assistance. ama. Part 2. Smithson. Misc. Collect. 150. The Condor90:933-935 0 The CooperOmithologd Society 1988 FEMALE PAIRING: A REPRODUCTIVE STRATEGY FOR HERRING GULLS? ’ GARY W. SHUGART~ Animal Behavior,Department of PsychologyNI-25, Universityof Washington,Seattle, WA 98195 MARY A. FITCH 5230 16th Ave NE, Seattle, WA 98105 GLEN A. Fox Wildrife Toxicologyand Surveys,Canadian Wildrife Service,Ottawa, Ontario KlA 0E7, Canada Key words: Larus argentatus;gulls: sex ratio; fe- METHODS male pairs; homosexual. Populationsof color-bandedindividuals at three Great Lakes island colonies were used for this study during Adaptive and nonadaptive explanations have been 1978 to 1986 (Table 1). Female pairs were identified provided for the phenomenonof female pairing in gulls by capturingsupernormal clutch attendantsand by lo- (Hunt and Hunt 1977, Coulson and Thomas 1985). In cating females that we had color-banded in previous this paper we summarize data for Herring Gull (Larus years.We confirmed that the supernormalclutches un- argentatus)female pairs that pertain to these interpre- der study were attended by female pairs and not by tations. one male-multiple female groups (Fitch and Shugart 1983, Fox and Boersma 1983) by observationsof nest attendants.At Lake Michigan colonies (see Shugartet al. 1987for locations),after banding,observations were I Received 8 December 1987. Final acceptance24 donefor a dawn to duskperiod then periodicallythrough June 1988. the breeding season(see Shugart 1980, Shugart et al. 2Present address:Department of Zoology, Univer- 1987). At Lake Ontario. attendantswere observeddur- sity of Oklahoma, Norman, OK 73069. ing nest checks. 934 SHORT COMMUNICATIONS TABLE 1. Summary of Herring Gull female pairs’ reproductive data. Study site Clutches (yearsof study) Clutch size examined Total eggs Egg status Gull Island’ 6 18 0 fertile (1981-1986) 5 25 0 fertile 4 4 0 fertile 3 6 0 fertile Hat Island2 6 6 0 fertile (1978-1979) 5 20 8 infertile, 12 undetermined Lake Ontario) 3 12 4 hatched, 8 undetermined (1979-1982) 2 4 0 hatched, 4 undetermined 1 0 hatched, 1 undetermined ’ Egg contentswere examined within a weekof laying. 1 Egg contentswere examined after the normal incubationperiod ’ Egg contentswere not examined. Identification of the sex of birds was done through mined fertility were fertile) to 96% (92 of 96) (assuming laparotomy in the initial years of study (five pairs at undetermined eggswere not fertile) (Table 1). The ap- Hat Island). We switched to measurements(Fox et al. parent inability of females to fertilize ova is a major 1981) after finding that laporotomy did not signifi- factor that limits Herring Gull female pairs’ reproduc- cantly increase accuracy in a large sample examined tive success. by Shugart(1980). In the initial years of the study, we determined re- productive successby monitoring nests for the normal DISCUSSION incubation periods. Contents of unhatched eggswere then examined at one colony. This technique did not The leading adaptive hypothesisfor female pairing is: allow accurateassessments of infertility, which we be- in specieswhere two adults are required to provide lieved was the causeof reproductive failure (Table 1). parental care (e.g., typical avian monogamy), under To obtain accurateinformation, we opened eggsand demographicconstraints of male shortages,females that examined blastodiscswith a lo-40 x microscope(see were unable to find male mates attempt to rear off- Fitch and Shugart 1983 for techniques). Addled eggs springcommunally as female pairs. Females could ob- were substitutedfor the eggsthat we opened. tain sperm from copulationswith paired males(Trivers 1972, Hunt and Hunt 1977, Fitch and Shugart 1984). RESULTS Three predictions that have been confirmed are: (1) femalepairs occur in colonieswith male shortages(Hunt FORMATION AND STABILITY OF PAIRS et al. 1980, Conover and Hunt 1984, Coulson and Seven females that were heterosexuallypaired in one Thomas 1985, Shugart et al. 1987) (2) female pairs year formed female pairs in the subsequentyear. Six form when a female has lost or cannot find a male (this ofthese formed three pairs after their male matesfailed paper), and (3) female pairs lay eggsand are therefore to return to the colony. The seventh paired with an attempting to reproduce (see below). unmarked female after her male paired with an un- A fourth prediction, that females should solicit cop- marked female. An eighth female paired with an un- ulations from paired males, is strongly supported by marked female in the year following a year as a non- the Ring-billed Gull (Lavusdeluwurensis). Female nairs breeding secondaryfemale (see Shugartet al. 1987). copulatedwith paired males and fertility ranged from Forty-two Herring Gull females (22 pairs) were col- 40-70% (Conover et al. 1979. review Kovacs and Rv- or-banded and followed for successiveyears. In the der 1983). The reproductive successper individual fk- year following a year of female pairing, 52% (22 of 42) male in female pairs was 20-27% of heterosexualfe- of the females paired with the same female, 2% (1 of males’ success(computed from table 5 and fig. 5, Kovacs 42) paired with an unmarked female, 2% (1 of 42) and Ryder 1983). Weaker support is provided by the remained in the vicinity of her territory as a floater low incidence of fertile eggs laid by female pairs of (Shugart et al. 1987) and 43% (18 of 42) were not Herring Gulls (4-30%, this paper), Western Gulls, Lar- observedin the colony. Although the stability of female usoccidentalis (13%, Hunt and Hunt 1977) and Black- pairs was lower than 93% interyear stability of Herring legged Kittiwakes, Rim triductylu (O%, Coulson and Gull heterosexualpairs (Fitch and Shugart 1983), the Thomas 1985). The latter may reflect a small sample 52% stability does suggestthat female pairs formed of eight eggs. “pair bonds” and they were not simply transient as- Based on our review of Ring-billed Gull data, and sociations. to a lesser extent the Western and Herring gull data, as yet, the hypothesis that female pairing is a repro- REPRODUCTIVE POTENTIAL ductive strategycannot be rejected. Even though the Herring Gull female pairs could potentially produce probability of reproductivesuccess is low, this minimal offspring becausethey lay eggs.Infertility ranged be- successcould produce a selective advantage in com- tween 70% (67 of 96) (assumingthat eggsof undeter- parison to zero success(Hunt and Hunt 1977, also see SHORT COMMUNICATIONS 935 Fisher 1958). Therefore adaptation remains as a viable FITCH, M. A., AND G. W. SHUGART. 1984. Require- competitor to mal-adaptive or neutral hypotheses. ments for a mixed reproductive strategyin avian species.Am. Nat. 124:116-126. We thank the U.S. Fish and Wildlife Service for Fox, G. A., AND D. BOERSMA.1983. Characteristics researchpermits on Gull Island, and G. L. Hunt, Jr. of supernormalRing-billed Gull clutchesand their and R. Macedo for reviewing the manuscript. This attendants.Wilson Bull. 95:552-559. researchwas supportedby a NSF doctoral dissertation Fox, G. A., C. R. COOPER,AND J. P. RYDER. 1981. grant (BNS-800-07582) to Fitch, by the Canadian Predicting the sex of Herring Gulls by using ex- Wildlife Service,and by a NSF postdoctoralfellowship ternal measurements.J. Field Omithol. 5 1:l-9. (BSR-8503050) to Shugart. HUNT, G. L., JR., AND M. W. HUNT. 1977. Female- LITERATURE CITED female pairing in Western Gulls (Larus occiden- talis) in Southern California. Science 196:1466- CONOVER,M. R., AND G. L. HUNT, JR. 1984. Exper- 1467. imental evidence that female-female pairs in gulls HUNT, G. L., JR., J. C. WINGF~ELD,A. NEWMAN,AND result from a shortageof breeding males. Condor D. S. FARNER. 1980. Sex ratio of Western Gulls 86472-476.