Ecological adaptation and birdsong: how body and bill sizes affect passerine sound frequencies

The avian bill is finely adjusted to foraging ecology and, as part of the vocal tract, it may also affect sexual signals such as songs. Acoustic theory predicts that larger bills lower the resonant frequency of vocal tracts, allowing larger-billed species to emphasize lower sound frequencies. Theory also predicts that identical changes in bill gape allow singing over a wider frequency bandwidth in larger-billed species. We tested these associations between bill size and sound frequencies of song, controlling for body mass, across ca. 1000 taxonomically-diverse passerines. Phylogenetically informed analyses indicated that both bill and body sizes are negatively related to the sound frequency of songs, with additive effects of similar strength. Analyses of reduced datasets, to decrease bill-body size associations, indicated that the effect of bill size remains identical and is thus not an artefact of its covariation with body size. Sound frequency bandwidth was only related to body size but not bill size, perhaps because large bills may allow greater modulation of frequency but also hinder fast bill movement. Since the bill has a major role explaining species differences in birdsong sound frequency, it can be a magic trait that promotes reproductive isolation as a consequence of ecological divergence.