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Sex Ratios of North American Gulls Based on Museum Collections

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Abstract

A survey of museum specimens revealed large interspecific variability in the sex ratio in 14 species [Mew, ring-billed, laughing, Franklin's, Bonaparte's, Heerman's, California, herring, Thayer's, western, glaucous, glaucous-winged, and great black-backed gulls, and blacklegged kittiwake] of North American gulls. In the three black-headed species, (Larus atricilla, L. pipixcan, and L. philadelphia), females outnumbered males among prefledglings, but males outnumbered females among adults. In the white-headed gulls, the male/female ratio in prefledglings did not vary significantly from 1.0 in any species. However, among first-year birds, females outnumbered males in 8 white-headed species. Among adults, the male/female ratio was < 1.0 in the Mew and Ring-billed gulls, > 1.0 in the Great Black-backed Gull, and = 1.0 in the other white-headed gulls.

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