Female-Female Pairs and Other Unusual Reproductive Associations in Ring-Billed and California Gulls

FEMALE-FEMALE PAIRS AND OTHER UNUSUAL REPRODUCTIVE ASSOCIATIONS IN RING-BILLED AND CALIFORNIA GULLS MICHAEL R. CONOVER, 1 •3 DON E. MILLER, I AND GEORGE L. HUNT, JR. 2 'Department of Zoology, Washington State University, Pullman, Washington 99164 USA, and 2 Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92717 USA ABSTRACT.-Most Ring-billed Gulls (Larus delawarensis) and California Gulls (L. californicus) form monogamous, heterosexual pairs during the breeding season. We report here the discovery of low frequencies (1-2%) of female-female pairings in Ring-billed and California gulls in three colonies in eastern Washington. The nests of these female-female pairs usually contained 5-6 eggs rather than the more normal 2-3. Between 60-70% of the eggs in the nests of the female-female pairs were fertile. We also found evidence of brood parasitism, which appears to be one cause of 4-egg clutches, and one case of polygyny in the Ring-billed Gulls. This polygynous association built a single communal nest that was incubated by three females and one male and apparently contained the eggs of at least two females. Received 13 November 1978, accepted 21 November GULLS characteristically form monogamous, heterosexual pair-bonds and lay 2- or 3-egg clutches (Paludan 1951, Tinbergen 1953). In the Western Gull (Larus oc- cidentalis), however, up to 14% of the nests from colonies in the California Channel Islands contain 4-6 eggs, most of which are infertile (Hunt and Hunt 1977). These supernormal clutches (4-6 eggs) apparently are laid by two females that share in- cubation and chick-rearing duties. Males are seldom observed at these nests. Su- pernormal clutches also occur in colonies of the Ring-billed Gull (L. delawarensis) and the California Gull (L. californicus), two species differing from the Western Gull in several aspects of their breeding biology. Whereas the Western Gull is non- migratory and spends much of the year on widely dispersed territories on the marine islands where it breeds, the Ring-billed and California gulls are migratory, stay in the nesting grounds only during the breeding season, and breed in dense colonies located on islands of inland lakes and rivers. METHODS On 12-13 May 1978 we surveyed three mixed-species colonies of Ring-billed and California gulls (Sprague Lake, Banks Lake and Potholes Reservoir colonies) in eastern Washington to determine the frequency of supernormal clutches, egg fertility, and female-female pairs. These colonies are described by Conover et al. (in press). Egg fertility was determined by opening eggs in the field and inspecting them for developing embryos or for lacunae in the blastodisc, which indicates infertility (Hunt and Hunt 1977). To determine the sex of birds associated with various sized clutches, adults of both species of gull were noosed on their nests (Miller 1974) at the Sprague Lake colony and sexed by laparotomy (Hunt and Hunt 1977). Sexed birds were banded, dye-marked or wing-tagged, and released. The nests occupied by these gulls were subsequently observed at repeated intervals over the next month to determine whether they were brooded and attended solely by the marked birds. RESULTS AND DISCUSSION We found that 3.3% of all Ring-billed Gull clutches examined contained more than 3 eggs (Table 1). Fertility in 5- and 6-egg clutches was less than that found in Present address: Department of Biology, Ball State University, Muncie, Indiana 47306 USA. The Auk 96: 6-9. January 1979