Comparative Behavior of Male-Female and Female-Female Pairs among Western Gulls Prior to Egg-Laying

The Condor 86:157-162 © Tbe Cooper Omj1hological Society 1984 COMPARATIVE BERA VIOR OF MALE-FEMALE AND FEMALE-FEMALE PAIRS AMONG WESTERN GULLS PRIOR TO EGG-LAYING G. L. HUNT, JR. A. L. NEWMAN M. H. WARNER J.C. WINGFIELD AND J. KAIWI ABSTRACT.-The behavior of male-female (M-F) and female-female (F-F) paired Western Gulls (Larus occidentalis) before egg laying was examined on Santa Barbara Island, California to test the hypothesis that one female in a F-F pair may assume a male role. In M-F pairs, no behaviors were performed exclusively by either sex, although males Mounted more often, females Head-tossed more often and males acted more aggressively toward intruders. Within F-F pairs, neither partner consistently showed masculine behavior. In a comparison of be- havior toward intruders, both members of F-F pairs resembled the female in M- F pairs, and were generally significantly different from males. We therefore reject the hypothesis that female-female pairing is the result of either the adoption of a male behavioral role by one or both partners, or an extreme female role by : one member. Female-female pairing in natural populations of birds was first documented in the Western Gull (Larus occidentalis) on Santa Barbara Is- land, California (Hunt and Hunt 1977), sub- sequently in the Ring-billed Gull (L. delawa- rensis) in the Great Lakes and in eastern Washington (Ryder and Somppi 1979, Con- over et al. 1979), and in the California Gull (L. californicus) in eastern Washington (Con- over et al. 1979). In these populations female- female pairing is associated with a frequency of super-normal clutches ranging from 1 to 14%, but this is only a partial index of the frequency of female-female pairs because some female-female pairs produce three-egg clutch- es. Additionally, abnormally large clutches have been reported for other species (Paludan 1951, Tinbergen 1960, Fordham 1964, Keith 1966, Klomp 1970) in which female-female partnerships might have been involved. Wingfield et al. (1980a, b) proposed two nonexclusive hypotheses concerning different aspects of the origin of these anomalous, though apparently widespread, female-female associ- ations: ( 1) Pairs of females may form because there are too few males of breeding age in the population. Hunt et al. ( 1980) found evidence of such a shortage among Western Gulls on Santa Barbara Island. (2) The process of mate selection by females (Tinbergen 1960, Brown 1967) may require that one of the females adopt male behavior patterns in order for pair for- mation to occur. We here test the second hy- pothesis, i.e., that female-female pairing in Western Gulls is facilitated because one of the females behaves like a male, thus effectively deceiving her mate. We tested the hypothesis of masculiniza- tion by comparing the behavior of female- female pairs (F-F) with that of male-female (M-F) pairs during the courtship and territorial period before egg laying. Were the hypQthesis correct, at least one member of each F-F pair should conform to the male stereotype. Al- though the territorial and breeding behavior of several species of gulls has been studied in detail (Moynihan 1955, 1958a, b; Tinbergen 1956, 1959, 1960; Vermeer 1963; Brown 1967; Burger and Beer 1975; Stout 1975), present knowledge of sexual differences in the behavior of large !arid species before egg laying (with the exception of Hand [1979) and Pierotti [ 1981]) is generally qualitative. Therefore, we examined quantitatively the qifferences in the roles of the partners in both M-F and F.-F pairs of Western Gulls during the pre-egg phase of the breeding season. METHODS Data were collected on the west side of Santa Barbara Island by five observers from 9 April to 17 May 197 6, and from 15 March to 30 May 1977 and in the spring of 1978. Of approximately 40 pairs observed in 197 6