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

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. 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. 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 values in forest species. Based on our results from song analyses, and the sender-receiver matching hypothesis, we predicted that open-habitat species would be more sensitive to high-frequency sounds compared to forest species. Contrary to this prediction, habitat and high-frequency song content had little effect on audiogram shape. Song type, however, had a strong effect, with species that produce complex songs showing higher sensitivity to high-frequency sounds than all other species. Our results suggest that the use of song frequency by receivers depends on song structure and not necessarily on song spectral content. Therefore, our current understanding of how signal-processing mechanisms should match signal properties appears to be too simple. When thinking about the evolution of signal-processing mechanisms, the multidimensionality of signals, and how the different dimensions can interact, should be considered.