Abstract
Animals use sounds to communicate in contexts that are crucial for survival and reproduction. One compelling feature of acoustic signals is that two main domains of variation-frequency and amplitude-can interact with each other for reasons both evolutionary and mechanistic, in ways that are framed by conflicting predictions. To explore this issue, we obtained amplitude-calibrated field recordings and assessed relationships between vocal frequency and amplitude in 53 species of birds. Our main findings are twofold. First, we identify considerable variation across species in their amplitude-frequency relationships, emerging as positive in 27 species, negative in 12 species and not discernible in 14 species. This variation, as well as results from phylogenetic models, suggests that amplitude-frequency relationships in birds are not governed by any universal rule. Second, throughout our sample-and particularly in oscines (songbirds)-we find that ranges of frequency variation are, by and large, amplitude-dependent, with 28 of our 35 most intensively sampled species producing broad ranges of frequencies at low amplitudes yet narrower ranges at high amplitudes. This finding is consistent with the idea that birds are constrained to produce high-amplitude songs only within the narrow ranges of frequencies that resonate most constructively within their vocal tracts-which, if upheld, could lead to intensity-complexity trade-offs at the song level.