*Heg, D. (1998) Personal communication.
Heg, D., B. J. Ens, T. Burke, L. Jenkins, and J. P. Kruijt (1993) “Why Does the Typically Monogamous Oystercatcher
*Heg, D., and R. van Treuren (1998) “Female-Female Cooperation in Polygynous Oystercatchers.”
*Makkink, G. F. (1942) “Contribution to the Knowledge of the Behavior of the Oyster-Catcher
*Nethersole-Thompson, D., and C. Nethersole-Thompson (1961) “The Breeding Behavior of the British Golden Plover.” In D. A. Bannerman, ed.,
*Nethersole-Thompson, D., and M. Nethersole-Thompson (1986)
Parr, R. (1992) “Sequential Polyandry by Golden Plovers.”
———(1980) “Population Study of Golden Plover
———(1979) “Sequential Breeding by Golden Plovers.”
Tomlinson, D. (1993) “Oystercatcher Chick Probably Killed by Rival Adult.”
GULLS AND TERNS
RING-BILLED GULL
IDENTIFICATION: A medium-sized (21 inch) gull with a gray back and wings; spotted black-and-white wing tips; yellow bill, legs, and eyes; and a black band on the bill. DISTRIBUTION: Central Canada, much of United States; winters south to Central America. HABITAT: Coasts, rivers, lakes, prairies. STUDY AREAS: Eastern Oregon and Washington State; Granite Island, Lake Superior; Ile de la Couvee, near Montreal; Gull Island, Lake Ontario; other locations on Lakes Erie, Ontario, and Huron.
COMMON GULL
IDENTIFICATION: Similar to Ring-billed Gull, except slightly smaller (up to 18 inches) and with a more slender, plain yellow bill. DISTRIBUTION: Nearly circumpolar in Northern Hemisphere; winters south to North Africa, East Asia, California. HABITAT: Coasts, mudflats, beaches, lakes. STUDY AREA: Fair Isle on the Shetland Islands, Scotland; subspecies
Social Organization
Common Gulls are fairly sociable, often associating in flocks of up to 100 individuals; sometimes tens of thousands of birds congregate outside of the breeding season. Ring-billed Gulls are also gregarious. Birds of both species generally form monogamous pair-bonds (although several variations exist—see below). Nesting colonies in Common Gulls contain a few dozen to several hundred pairs, while Ring-billed colonies can be much larger, in the tens of thousands of pairs.
Description
The first breeding season that female Common Gulls begin a pair-bond, they may build a “double nest” consisting of two separate but touching nest cups; in subsequent years, they will build only a single nest in which they both lay eggs (like most Ring-billed female pairs). Since both partners usually lay eggs, nests of homosexual pairs often contain “oversize” or SUPERNORMAL CLUTCHES of 5—8 eggs (Ring-billed Gull) and 6 eggs (Common Gull)—up to twice the number found in heterosexual nests. Some Ring-billed Gulls in female couples may also lay their eggs in the nests of other (heterosexual) pairs. One or both partners in pairs of female Ring-billed Gulls may mate nonmonogamously with a male so that some of their eggs will be fertilized. Female Common Gulls in bisexual trios can also lay fertile eggs by mating with their male partner. Both females share incubation duties and also cooperate in parenting the chicks that they hatch. Homosexual parents in Ring-billed Gulls invest as much time as their heterosexual counterparts do in feeding their chicks, spending time on the nesting territory, and defending their territory. They may actually work harder than male-female pairs in brooding and defending their chicks, with the result that offspring of female pairs often have a faster growth rate than chicks of heterosexual parents.
Nevertheless, chicks belonging to female pairs are often less robust on hatching, and female parents generally raise less than half the number of chicks that male-female pairs do. However, these traits are also characteristic of supernormal clutches belonging to heterosexual trios and are therefore undoubtedly related to the larger clutch size rather than the sex or abilities of the parents. In addition, in some populations female pairs are relegated to smaller, substandard territories on the periphery of the breeding colony or in between other territories (as are less experienced heterosexual pairs). In some cases, homosexual pairs actually appear to form clusters of up to ten nests in close proximity, or else small groups of two or three (sometimes in a straight-line or triangular formation). Many of these nests are located in areas where it is more difficult to parent successfully—places with little or no vegetation, or else away from the beach—and therefore it is remarkable that female pairs are able to successfully raise chicks as often as they do in such less than optimal conditions.