Territoriality modulates the coevolution of cooperative breeding and female song in songbirds

Birdsong has historically been characterized as a sexually selected, primarily male behavior. More recent findings suggest female song is widespread, raising questions about how social functions of birdsong shape song evolution. Certain breeding systems, like cooperative breeding, change social dynamics and selection pressures on both sexes, providing important contexts for studying song evolution. Here, we use phylogenetic comparative analyses across 1041 songbird species to examine relationships between cooperative breeding, female song, and male song characteristics. Here, we show robust bidirectional co-evolutionary dynamics between cooperative breeding and female song that persist when controlling for territoriality, allometry, and geographic sampling biases. Importantly, when examining intensity of territorial defense, we find this relationship is context-dependent: while cooperative breeding and female song commonly co-occur in strongly territorial systems, their association is especially pronounced in weakly territorial systems, where they co-occur much more often than expected by chance. Additionally, we observe that male song repertoire size evolves more slowly in cooperative-breeding lineages. These findings demonstrate that cooperative breeding shapes the evolution of vocal communication differently based on territorial context and sex, with female song potentially serving crucial but understudied functions related to social cohesion in cooperative systems, particularly in species where territorial conflict is reduced.