Acoustic signals mediate key animal interactions and can evolve through a variety of factors. Signal divergence can reinforce pre -zygotic barriers and minimize costly hybridizations among closely related species or partition acoustic space to avoid signal interference. To unravel the drivers of song evolution, it is critical to simultaneously test multiple evolutionary axes leading to heterospecific song variation (e.g., the role of morphology, ecology, and heterospecific recognition). Tyrannidae is the largest Passeriformes family and occurs across a broad environmental gradient. Tyrannids are suboscines, so song variation represents evolutionary differences that are not confounded by social learning. Several genera show conserved plumage coloration, but exhibit pronounced vocal differences. In the first of our two-part paper on signal ecology and evolution, we leveraged a large-scale song dataset to unravel song divergence in Tyrannidae (n = 282 species) by testing the relative influence of morphology (body and bill size), ecology (vegetation cover, diet, climate), and heterospecific proximity on song evolution. Based on phylogenetically controlled analyses, we found Tyrannidae song evolution was primarily driven by morphological adaptation, where larger-bodied birds with heftier bills sang lower frequency and slower paced songs. Pairwise song differences were weakly related to heterospecific proximity of some genera, lending support to the species recognition hypothesis or drift. Given that many flycatchers are habitat specialists, natural selection acting on bill morphology and body size in specific environmental-contexts may shape song among tyrannids. By simultaneously testing the relative roles of morphological, ecological, and geographical factors on song evolution, our study highlights the complexity of suboscine song evolution and the importance of large-scale comparative studies that test multiple evolutionary hypotheses.
