Evolutionary drivers of vocal amplitude in birds.

Vocal amplitude is crucial for acoustic communication because it determines how far a sound travels and can convey valuable information about signal senders to signal receivers. Yet, until recently, amplitude had received limited attention in the literature mainly because of operational challenges in its measurement. As a result, fundamental questions about amplitude remain largely unanswered even in one of the most extensively studied group of singers-birds. Here, we set out to answer two such questions: to what degree do birds vary in how loud they can vocalize, and what evolutionary factors drive this variation? Using original amplitude data from 123 bird species, we show that maximum measured amplitudes vary broadly across species-from 64 dB LAeq,20ms at 1 m in a species of hummingbird to 122 dB in a species of cotinga-, with two new species emerging as candidates for the world's loudest bird (as yet identified). Regarding evolutionary drivers of amplitude, we found that species with larger body sizes and larger beak gape widths tend to achieve higher maximum amplitudes. By contrast, ecological predictors assessed-aerial mobility, habitat density, and mating system-had no unequivocal effect on maximum amplitude. By establishing connections between morphology and vocal amplitude, our results represent a step forward in uncovering what enables birds to produce the loudest acoustic signals of all terrestrial fauna.