The evolutionary origin of variation in song length and frequency in the avian family Cettiidae

Aspects of bird song have been shown to correlate with morphological and ecological features, including beak and body size, and habitat. Here we study evolution of song length and song frequency among 30 species belonging to the Cettiidae. Frequency is negatively correlated with body size, and song length increases with latitude. Although migration distance correlates with latitude, the association of song length with latitude is only present within the non-migratory species, implying the association is not a consequence of migration. We place these correlations in a historical framework to show that the body size-frequency association arose early in the group, but the latitude-song length association is more evolutionarily labile. We suggest that latitudinal correlates of song length may reflect increased importance of sexual selection by female choice.