Song structure, not high-frequency song content, determines high-frequency auditory sensitivity in nine species of New World sparrows (Passeriformes: Emberizidae)

1. The evolution of vocal signals can be constrained by a host of factors including habitat effects on sound propagation, morphology of sound-producing structures, and phylogenetic relationships among species. Here, we asked whether auditory sensitivity over a broad range of frequencies correlates with the spectral content of conspecific vocalizations, or whether it is constrained by the overall structure of vocalizations, habitat effects on sound propagation, or relatedness among species. 2. We studied nine New World sparrows (Passeriformes: Emberizidae) including three open-habitat species, three scrub-like habitat species, and three forest species. For each habitat, one species had pure trilled songs, another had tonal songs, and another had complex songs with tones, trills, and amplitude-modulated buzzes. 3. As predicted by the acoustic adaptation hypothesis, song spectral properties (specifically frequency and entropy) had the highest values in open-habitat species and the lowest v...