Abstract
Despite growing recognition that female bird song is widespread, large-scale comparative tests of the evolutionary drivers of female song remain rare. Most conceptual frameworks for studying birdsong have been developed and tested primarily using male traits, leaving open the question of whether ecological, morphological, and social pressures shape song evolution in females. Here, we apply phylogenetic comparative methods to test three classic hypotheses of signal evolution—acoustic adaptation, morphological adaptation, and species recognition—using female song data from 197 species of antbirds (Thamnophilidae), a socially monogamous Neotropical clade with widespread female vocalizations. We found that habitat structure and morphological traits constrain multiple aspects of female song. Species in unexposed (i.e., closed) habitats produced songs with fewer notes, slower pacing, and lower vocal performance, consistent with the acoustic adaptation hypothesis. Larger-bodied females with larger bills produced longer songs with lower peak frequency, narrower bandwidth, and lower vocal performance, supporting the morphological adaptation hypothesis. These results indicate that female song diversification is shaped by ecological and morphological constraints. We also found that syntopic species pairs—those that overlap in both habitat and elevation—exhibited reduced vocal similarity, supporting the species recognition hypothesis. However, divergence time also explained some variation in vocal similarity between syntopic pairs, indicating that both ecological interactions and evolutionary history contribute to female song divergence. Altogether, these results provide a rare comparative test of classic signal evolution hypotheses using female song. This study provides the first macroevolutionary test of multiple hypotheses of signal evolution in female birdsong. By showing that female song is shaped by both ecological and morphological constraints and diverges slightly more among syntopic species, our results emphasize the need to integrate female traits into evolutionary models of communication. Incorporating female perspectives will be essential to building a complete understanding of signal diversity and its evolution across species.