Bill size, bill shape, and body size constrain bird song evolution on a macroevolutionary scale

Studying macroevolutionary patterns of phenotypic variation and their driving forces in large radiations can shed light on how biodiversity is generated across broad spatiotemporal scales. In this study, we integrated song and morphological variation across more than 300 species representing the largest family of songbirds, the tanagers (Thraupidae), to uncover how morphological variables of the vocal tract combine to shape vocal evolution on a macroevolutionary scale. We found that body size correlated with multiple frequency parameters, concurring with past studies that show how body size constrains vocal evolution. Furthermore, bill size predicted multiple frequency and temporal song characters while bill shape was strongly associated with trill rates, suggesting that bill size and shape both constrain distinct elements of avian song independently of body size covariation. Our results demonstrate that the relationship between morphology and song exhibits modular variation when expanded to a macroevolutionary scale. Furthermore, our study emphasizes the need to consider multiple components of the avian vocal tract when exploring the macroevolutionary interplay of morphological traits and acoustic signals. LAY SUMMARY Avian vocalizations vary widely among birds and are associated with multiple aspects of their biology, including mechanisms of sexual selection, habitat acoustics, and avian vocal tract morphology. The avian vocal tract is composed of many internal and external traits, but relatively few studies have disentangled the impact of multiple avian tract components on the evolution of bird song. In this study, we quantify associations between 11 song variables and tarsus length (representing body size), bill size, and bill shape used in courtship displays of tanagers, the largest family of songbirds. We find that both body size and bill morphology (size and shape) are correlated with various aspects of tanager song. Taken together, different components of the avian vocal tract selectively constrain unique aspects of their vocalizations. Future studies could examine internal (e.g., tracheal and syringeal) morphological traits in combination with bill morphology to gain a more comprehensive insight into how different elements of the avian vocal tract shape avian vocalizations.