Sex roles in birds: Phylogenetic analyses of the influence of climate, life histories and social environment.

Sex roles describe sex differences in courtship, mate competition, social pair-bonds and parental care. A key challenge is to identify associations among the components and the drivers of sex roles. Here, we investigate sex roles using data from over 1800 bird species. We found extensive variation and lability in proxies of sex roles, indicating remarkably independent evolution among sex role components. Climate and life history showed weak associations with sex roles. However, adult sex ratio is associated with sexual dimorphism, mating system and parental care, suggesting that social environment is central to explaining variation in sex roles among birds. Our results suggest that sex differences in reproductive behaviour are the result of diverse and idiosyncratic responses to selection. Further understanding of sex roles requires studies at the population level to test how local responses to ecology, life histories and mating opportunities drive processes that shape sex role variation among higher taxa.