Abstract
Investigations of the colonies of western gulls on Santa Barbara Island, California, have revealed a surplus of females and the occurrence of female-female pairs that produce clutches with as many as six eggs. Females are able to establish and defend breeding territories, behaviours generally thought to be under the control of androgens. There are very few significant differences in circulating levels of the luteinizing hormone and androgens among breeding males, breeding females in heterosexual pairs, and breeding females in homosexual pairs. In contrast, however, only females sampled in 1877, in both homo- and heterosexual pairs, have elevated plasma levels of oestrogens, in spring, coincident with the period in which they show courtship behaviours such as food begging and solicitation of copulation. Given a sex ratio skewed in favour of females, as is apparently the case with the colonies of this species on Santa Barbara Island, and the essentially equal plasma levels of androgens in males and females, it is not difficult to rationalize the formation of female-female pairs. Our findings do not support the hypothesis that female-female pairing involves hormonal masculinization of one member of the pair.
Highlights
It is for this reason that it seemed possible that one of the females of the homosexual pair had undergone endocrine masculinization
The major focus of the study was on the colony on the northwest slope of Santa Barbara Island
Changes in diameter of ovarian follicles, plasma levels or immunoreactive LH (irLH) and steroid hormones of homosexually paired female western gulls are presented in Fig. 4. (See Table V for explanation of this figure.) We found no major differences between homosexuallyand heterosexuallypaired females in either the concentrations of hormones in the plasma or in their temporal patterns during the breeding season, with the exception of the period of incubation in which circulating irLH continued to increase in homosexually paired females (Fig. 4, P < 0.01) and to decrease in heterosexually paired females (Figs 1 and 3)
Summary
Gulls followed the bait into the trap readily, and once inside appeared unable to locate the small inner entrance and were trapped This technique was useful during the preand post-nesting phases of the cycle and for capture of unmated or non-nesting birds. A preliminary analysis of changes in hormone levels during the period of sampling revealed that there is a decrease in plasma LH and steroid hormones 30 to 60 rain after capture. In 1977 daily censuses were taken of colour-ringed birds in the colony and observations were made on their reproductive status and stage in the cycle. In 1978 we made regular censuses, recorded reproductive status and collected blood samples from colour-ringed, unpaired individuals for which we had determined sex by earlier laparotomy. Comparisons between sexes were made with the Student's t-test
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