Adult sex ratio variation and its sex-specific predictors in shorebirds

The proportion of adult males to females in the adult population, the adult sex ratio (ASR), is an important demographic parameter that has implications in sexual selection, ecology and conservation. ASR variation can be multifactorial but specific variables including sex roles - sex differences in courtship, mate competition, social pair-bonds and parental care- and sex-specific mortality have been suggested as important ASR determinants in birds. However, these relationships have not yet been comprehensively tested in specific avian groups. Here, we used phylogenetic comparative methods to study drivers of ASR variation across shorebirds (Charadrii and Scolopaci; n = 205 species), a charismatic bird group characterised by displaying extreme variations in ecology, sex roles and sexual selection traits. We found that ASR variation is associated with most sex role components but not with their sex-specific mortality. Although sex role and life history variables showed no significant sex bias, we found a trend towards reversed size dimorphism and sex role reversal across shorebirds. Sex roles components also showed correlations among each other that were surprisingly strong and in unexpected directions. Our results confirm that sex roles are important drivers of ASR variation and suggest that shorebirds may have alternative means of sex-specific mortality, possibly linked to their ecology.