Bird song has served as a model system for the study of the evolution of vocal communication. Several studies have uncovered factors that regulate the evolution of species-specific song types. For instance, closely related species have similar anatomical features (such as the size of the bill in songbirds) that can constrain the types of song a bird can produce. Similarly, habitat is an important factor that constrains the use of certain song properties. Birds that live in forests tend to have tonal songs with relatively low frequencies: these are properties that facilitate propagation through the understory. In contrast, birds that live in open areas tend to have songs with relatively higher frequencies because there are fewer constraints on sound propagation in open habitats.
Effective communication should result in a close match between bird song characteristics and the auditory mechanisms that facilitate signal processing, i.e. birds should be good at hearing the kinds of songs they make. The authors asked whether hearing sensitivity correlates with song and habitat characteristics in nine species of sparrows. The authors studied three species that inhabit forest areas, three species that live in intermediate, scrub-like areas, and three species that live in open areas. They measured song frequency content and hearing sensitivity over a broad range of frequencies. Based on previous studies, we predicted higher song frequencies in open-habitat species than in forest species. Based on the hypothesis that hearing correlates with song characteristics, we predicted higher sensitivity to high-frequency sounds in open-habitat species than forest species.
Consistent with previous studies, song frequency was highest in species from open habitats and lowest in forest species. Contrary to predictions, however, song frequency and habitat were not correlated with high-frequency hearing sensitivity. Instead, overall song structure was highly correlated with high-frequency hearing sensitivity. Species with structurally complex songs (like the song sparrow) were more sensitive to high-frequency sounds than species that produce tonal songs (like the white-throated sparrow), or trilled songs (like the dark-eyed junco). These results highlight the importance of considering the different dimensions of vocal signals when we think about the evolution of sensory mechanisms that allow for signal processing.