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FEMALE-BIASED SEX RATIO IN A WINTERING POPULATION OF RUBY-CROWNED KINGLETS

Abstract Many avian species have demonstrated differential wintering of the sexes, either latitudinally across their wintering range or by habitat. We examined the large numbers of wintering Ruby-crowned Kinglets (Regulus calendula) captured as part of a long term constant effort mist-netting program in coastal California to determine if such a pattern was evident in this population. Data from 1976 to 1997 revealed a consistent and significant female bias, with a mean of 2.13 (± 0.31 SE) females caught per male. We evaluated the possibility that this bias resulted from differential capture probabilities of the sexes, and found no difference in recapture probabilities between males and females. The sex ratio was biased toward females in both scrub and mixed evergreen forest habitats, but significantly more skewed in the former (3.68:1 versus 1.91:1). We also examined captures over a two-year period from nearby stations in riparian forest; the sex ratio was least skewed in this habitat (1.37:1). Considered together with the latitudinal differential distribution of this species observed in eastern and central North America, our data suggest that Ruby-crowned Kinglets may not only be geographically but also ecologically segregated according to sex across their wintering range.