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 larger-billed species can change bill gape so as to sing over a wider frequency bandwidth. We tested these associations between bill size and sound frequencies of song 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 it is 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 m...