The evolution of extreme sound frequencies in bird songs

Bird songs differ widely among species and can show peculiar phenotypes, such as extreme or unusual sound frequencies for a species' body size. Although birds modulate sound frequency, size-related limitations prevent vocalizing efficiently (i.e., with high amplitude) at any frequency. To understand how the evolution of unusual sound frequencies interacts with constraints on sound amplitude, we compared where peak amplitude is located within the frequency bandwidth of songs (hereafter PRRR: peak relative to realized range) across >1,000 passerine species. Consistent with constraints on sound amplitude increasing toward the song bandwidth fringes of each species, PRRR was usually close to the bandwidth midpoint, and very few species had PRRR close to their upper or lower bandwidth limits. We found that constraints on amplitude often evolved such as to facilitate singing extreme sound frequencies: on average, species using higher-frequency frequency ranges than expected for their body size had higher PRRR compared to species with lower-frequency frequency ranges than predicted for their size. This indicates that, despite constrained by size, the evolution of unusual or extreme sound frequencies is often accompanied by adaptations (e.g., morphology of the vocal organ or tract) that to some extent facilitate singing at those unusual frequencies.