Global patterns and evolution of avian song complexity

Why birds evolved complex songs has been a long-standing puzzle. Here, by quantifying song complexity in 4,940 species (83% of all passerines), we show a latitudinal gradient in song complexity, with the most complex songs occurring in temperate, open-habitat bird assemblages characterized by relatively inconspicuous plumage. Using comparative analyses controlled for common ancestry, we find little support for traditional explanations based on sexual selection intensity, intelligence signaling, or species richness. Instead, complex songs evolve primarily in song-learning passerines, especially migrants, habitat generalists, and species breeding in open habitats where structured acoustic signals transmit more efficiently. Together, our findings indicate that global variation in song complexity emerges from the interaction between vocal learning, ecological context, and evolutionary history rather than from a single selective driver.