Disentangling the Evolutionary Cause-Effect Relationships of Environment, Sexual Selection, and Body Size With Birdsong Frequency.

Multiple selection pressures act simultaneously and interactively to shape trait evolution, yet their combined effects are often overlooked. We used Phylogenetic Path Analysis to examine the causal relationships between birdsong frequency, body size, habitat structure, and sexual selection across 472 Neotropical passerine species. This approach quantifies direct and indirect effects while comparing alternative evolutionary scenarios. A single causal model consistently explained variation in song frequency across 100 phylogenies. Tree cover increased minimum, peak, and maximum frequencies but did not affect bandwidth. Sexual dimorphism reduced bandwidth and influenced frequency, whereas morphological traits constrained frequency parameters and differentially shaped bandwidth. Habitat structure and sexual dimorphism also affected morphology, generating additional indirect effects on song frequency, and tree cover further influenced sexual dimorphism. Together, these results reveal an integrated network of environmental, sexual, and morphological drivers of acoustic evolution, supporting hypotheses of sexual selection, acoustic adaptation, and morphological constraints. Our findings highlight that trait evolution emerges from interacting processes, and that simple linear models may fail to capture the complexity of these evolutionary pathways.