females, however, there is further variation. In some same-sex couples, both females mate with males and lay fertile eggs; in others, only one partner does, or each partner might lay both fertile and infertile eggs at different times, indicating temporal variation in bisexual activity. Similarly, in a Common Gull bisexual trio, one female remained exclusively homosexual even though her female partner mated with the male. In addition, a large number of “heterosexual” Ring-billed females may have a “latent” bisexual potential, since many are able to develop bonds with other females if single males are not available.
Nonreproductive and Alternative Heterosexualities
Heterosexual pairs in Ring-billed and Common Gulls exhibit a variety of bonding and parenting arrangements (like homosexual pairs). Not all males and females couple for life: the heterosexual divorce rate is about 28 percent in both species. Polygamous heterosexual trios—two females bonded to the same male, but not to each other—are also found in both species, as are occasionally even quartets (three females with one male). Common Gull pairs sometimes foster-parent chicks, while another form of “adoption” occurs in these species when females occasionally lay eggs in nests belonging to other pairs or roll eggs from other nests into their own. Moreover, because of parental ineptitude or inefficiency (such as poor feeding), at least 8 percent of Ring-billed chicks abandon or “run away” from their own families; most of these are adopted and cared for by other families.
About 4 percent of Ring-billed pairs continue to engage in courtship and copulation after the hatching of their eggs—when sexual activity is not directly reproductive—and about 5 percent of adults court and mount chicks. Most of this activity involves females behaving incestuously with their own offspring, including full copulatory REVERSE mounts of young birds. Mounted chicks may be as young as two weeks old, and they usually collapse under the weight of the adult mounting them and cry out in distress. Some individuals appear to be “habitual molesters” in that they repeatedly interact sexually with chicks, including their own. In addition to sexual molestation, Ring-billed chicks are often subjected to vicious attacks from neighboring adults when their parents are away, or if they stray outside of their home territory. About 1 in 300 chicks is killed by such assaults, and infanticide can account for between 5 percent and 80 percent of all chick deaths (depending on the population).
Other Species
Female pairs that lay supernormal clutches also occur in California Gulls (Larus
Brown, K. M., M. Woulfe, and R. D. Morris (1995) “Patterns of Adoption in Ring-billed Gulls: Who Is Really Winning the Inter-generational Conflict?”
*Conover, M. R. (1989) “Parental Care by Male-Female and Female-Female Pairs of Ring-billed Gulls.”
*———(1984a) “Frequency, Spatial Distribution, and Nest Attendants of Supernormal Clutches in Ring-billed and California Gulls.”
*———(1984b) “Consequences of Mate Loss to Incubating Ring-billed and California Gulls.”
*———(1984c) “Occurrence of Supernormal Clutches in the Laridae.”
*Conover, M. R., and D. E. Aylor (1985) “A Mathematical Model to Estimate the Frequency of Female-Female or Other Multi-Female Associations in a Population.”
*Conover, M. R., and G. L. Hunt, Jr. (1984a) “Female-Female Pairings and Sex Ratios in Gulls: A Historical Perspective.”
*———(1984b) “Experimental Evidence That Female-Female Pairs in Gulls Result From a Shortage of Males.”
*Conover, M. R., D.E. Miller, and G. L. Hunt, Jr. (1979) “Female-Female Pairs and Other Unusual Reproductive Associations in Ring-billed and California Gulls.”
Emlen, J. R., Jr. (1956) “Juvenile Mortality in a Ring-billed Gull Colony.”
Fetterolf, P. M. (1983) “Infanticide and Non-Fatal Attacks on Chicks by Ring-billed Gulls.”
———(1984) “Ring-billed Gulls Display Sexually Toward Offspring and Mates During Post-Hatching.”
*Fetterolf, P. M., and H. Blokpoel (1984) “An Assessment of Possible Intraspecific Brood Parasitism in Ring- billed Gulls.”
*Fetterolf, P. M., P. Mineau, H. Blokpoel, and G. Tessier (1984) “Incidence, Clustering, and Egg Fertility of Larger Than Normal Clutches in Great Lakes Ring-billed Gulls.”
*Fox, G. A., and D. Boersma (1983) “Characteristics of Supernormal Ring-billed Gull Clutches and Their Attending Adults.”
Kinkel, L. K., and W. E. Southern (1978) “Adult Female Ring-billed Gulls Sexually Molest Juveniles.”
*Kovacs, K. M., and J. P. Ryder (1985) “Morphology and Physiology of Female-Female Pair Members.”
*———(1983) “Reproductive Performance of Female-Female Pairs and Polygynous Trios of Ring-billed Gulls.”
*———(1981) “Nest-site Tenacity and Mate Fidelity in Female-Female Pairs of Ring-billed Gulls.”
*Lagrenade, M., and P. Mousseau (1983) “Female-Female Pairs and Polygynous Associations in a Quebec Ring-billed Gull Colony.”
Nethersole-Thompson, C., and D. Nethersole-Thompson (1942) “Bigamy in the Common Gull.”
*Riddiford, N. (1995) “Two Common Gulls Sharing a Nest.”
*Ryder, J. P. (1993) “Ring-billed Gull.” In A. Poole, P. Stettenheim, and F. Gill, eds.,
*Ryder, J. P., and P. L. Somppi (1979) “Female-Female Pairing in Ring-billed Gulls.” Auk 96:1—5.
Southern, L. K., and W. E. Southern (1982) “Mate Fidelity in Ring-billed Gulls.”
Trubridge, M. (1980) “Common Gull Rolling Eggs from Adjacent Nest into Own.”
WESTERN GULL
IDENTIFICATION: A large gull (up to 27 inches) with a dark gray back and wings; spotted black- and-white wing tips; pink legs; and a yellow bill with a red spot. DISTRIBUTION: Pacific coast of North America. HABITAT: Cliffs, rocky seacoasts, bays. STUDY AREAS: Santa Barbara Island and other Channel Islands, California; subspecies
KITTIWAKE
IDENTIFICATION: A smaller gull (to 17 inches) with a blue-gray mantle; more pointed black wing tips; relatively short black legs and dark eyes; and a yellowish green bill. DISTRIBUTION: Northern Pacific and Atlantic Oceans; adjacent Arctic Ocean. HABITAT: Oceangoing; breeds on coasts. STUDY AREA: North Shields, Tyne and Wear, England; subspecies
