EXPERIMENTAL-EVIDENCE THAT FEMALE-FEMALE PAIRS IN GULLS RESULT FROM A SHORTAGE OF BREEDING MALES

The Condor 8 6:472~ 76 © The Cooper Ornithological Society 1984 EXPERIMENTAL EVIDENCE THAT FEMALE-FEMALE PAIRS IN GULLS RESULT FROM A SHORTAGE OF BREEDING MALES MICHAEL R. CONOVER AND GEORGE L. HUNT, JR. ABSTRACT.-We tested the hypothesis that female-female pairings in Ring- billed (Larus delawarensis) and California gulls (L. californicus) result from a shortage of males in the breeding population. This hypothesis was tested by removing males from one Ring-billed and four California gull colonies early in the breeding season. The frequency of 4-6 egg clutches, which we used as an index offemale-female pairings, was significantly higher in these colonies than in nearby control colonies, thus supporting our hypothesis. Our results indicate that female- female· pairings allow females the chance to breed when they are unable to obtain a male partner. Although terns and gulls are usually monog- at some small colonies of Ring-billed and Cal- amous, female-female pairings have recently ifornia gulls. By doing so, we hoped to answer been discovered in Caspian Terns (Sterna cas- the question of why two females would pair pia; Conover 1983), Western Gulls (Larus oc- together rather than with male mates. Previous cidentalis; Hunt and Hunt 1977), Ring-billed authors have suggested that mating systems Gulls (L. delawarensis; Ryder and Somppi are either unrelated to sex ratios (Verner 1964; 1979, Conover et al. 1979a), California Gulls Witten berger 197 6, 1979) or influence sex ra- (L. californicus; Conover et al. 1979a) and in tios (Hamilton 1967, Wilson and Colwell the Great Lakes population of Herring Gulls 1981 ). In the case of these gulls, we were testing (L. argentatus; Fitch 1980). Both females in a the hypothesis that sex ratios determine the same-sex pair defend a single territory and lay mating system used. eggs in the same nest, often resulting in a su- pernormal clutch of 4-6 eggs, double the nor- METHODS mal number of 2-3. Both females incubate and If a shortage of males in a breeding population leads to female-female pairings, then removing share other parental responsibilities. Much interest has centered on why one fe- males from the colony should lead to an in- male would pair with another, because the re- crease in the frequency of such pairings in col- productive success of these females is often onies where female-female pairs already occur. much lower than that of heterosexually-paired In 1981 we removed males from one Ring- females (Hunt and Hunt 1977). One hypoth- billed Gull colony and four California Gull esis (Wingfield et al. 1980a, b, 1982) is that colonies; we compared the frequency of fe- the behavior results from a pairing preference male-female pairings in these colonies to those of some females for the wrong sex, perhaps in nearby (control) colonies where no males owing to behavioral or endocrine masculini- were removed. All colonies were located in zation of these females. Wingfield et al. ( 1980a, Washington and Oregon and are described b, 1982) and Hunt et al. ( 1984), however, found elsewhere (Conover et al. l 979b). We captured gulls from the experimental no significant hormonal or behavioral differ- ences between homosexually- and heterosex- colonies by first baiting them on islands lo- cated within 100-200 m of the colony and in ually-paired females in Western Gulls. A second hypothesis proposes that female nearby garbage dumps, and then firing a rocket gulls pair together when they are unable to net over them. All trapping was done early in obtain male mates, owing to a shortage of the 1981 breeding season (March and April), breeding males (Hunt and Hunt 1977, Con- when gulls were starting territorial defense and over et al. 1979a, Fry and Toone 1981, Pierotti courtship but before they began copulating. We captured gulls at six sites close to the 1981). This hypothesis was supported by the observation that females outnumbered males four experimental colonies but not inside them. three-to-two in a Western Gull colony where Prior experience had taught us that if in-colony female-female pairings occur (Hunt et al. 1980). capture were attempted before the incubation We tested this hypothesis by experimentally period, many of the gulls would desert the col- manipulating the sex ratio of breeding adults ony. Birds from the Potholes Reservoir colony