The comparative effects of small geographic range and population decline on the adult sex ratio of threatened bird species

Abstract

Although the factors associated with adult sex ratio (ASR) skew in threatened species are rarely identified, ASRs of threatened species appear to be more male-skewed with increasing severity of threat. In this study we investigate whether the ASRs of species classified as threatened because of decline are significantly different from those threatened because of small range. Despite previous studies suggesting an association between male-skewed ASRs and population decline, our results show that ASRs may be more male-skewed in species classified as threatened because of small range. Although selection would be expected to purge dispersal genotypes from isolated populations, our finding could result from an imbalance between immigration and emigration rates of species with small ranges. Future research should examine rates of emigration and immigration in species with small global ranges.