Sex differences in singing behaviour are predicted by territoriality and biparental care in songbirds

Pronounced sexual dimorphism is thought to evolve through sexual selection for elaborate male traits. Increasing evidence suggests that sexual dimorphism in traits such as birdsong may also evolve through loss of elaboration in females, but the evolutionary drivers underlying this are obscure. Here we analyse ecological and natural history traits of over 1300 songbird species and show that increased female song incidence and elaboration are most directly associated with year-round territoriality, biparental care, and large body size. Moreover, phylogenetic path analysis indicates that mating system and breeding latitude have only indirect effects. Stable, tropical life histories and mating systems with biparental care promote female song, whereas evolutionary transitions to migration, reduced territoriality, and loss of male care led to losses of female song. Our results provide the first comprehensive framework to understand the drivers of sex differences in birdsong and reveal novel interactions among natural history, social, and sexual selection pressures in the evolution of avian song in both sexes.