Female-female pairing and nests with supernormal clutches @NC) of eggs occur in gull breeding colonies when two conditions are met: (1) sex ratio of breeding adults is skewed to females; and (2) nest sites are available for female-female pairs or polygynous trios. Expanding gull populations forming new colonies have SNC arising presumably from greater dispersion of females from their natal site. Declining populations in areas polluted with organochlorines (OC) show a decrease in breeding males and up to 15% of SNC. Population change has been predictive of both SNC incidence and sex ratio skew. SNC frequency and historical occurrence parallel high levels of OC pollution in southern California and the Great Lakes. OC exposure comparable to southern California has been experimentally duplicated by injecting OC into gull eggs, causing abnormal development of both male and female embryos. Males are feminized with germ cells located in the cortex of the gonad. Females develop both right and left oviducts. Persistent right oviducts, eggshell thinning and SNC in populations of gulls in Puget Sound, WA indicate recent exposure to estrogenic pollutants.
Citation:
Fry, D.M., C.K. Toone, S.M. Speich, and R.J. Peard. 1987. Sex Ratio Skew and Breeding Patterns of Gulls: Demographic and Toxicological Considerations. Studies in Avian Biology 10: 26-43.
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