The Role of Body Size, Phylogeny, and Ambient Noise in the Evolution of Bird Song

Morton found that the average emphasized frequency in the songs of tropical bird species in low-forest habitats was lower than that of species in grassland or edge habitats. He suggested that this was due to a frequency window around 1585-2500 Hz in low-forest habitats, and that there was selection to produce songs with frequencies in this window in order to increase the transmission distance of the songs. In the present study, we analyze the constraints of body size and evolutionary history on the ability of avian species to respond to this selection. Also, we examine the spectral distribution of ambient noise as an additional selective factor. There is a correlation between body size and the emphasized frequency of the song for the species analyzed by Morton. Larger birds produce songs with lower emphasized frequencies. Morton's demonstration that birds in the low forest produce songs with lower frequencies is confounded by the fact that larger birds also live in this habitat. Phylogeny is also a potentially confounding factor, since the allometry of size and frequency differs among taxonomic units. Our analyses, however, control for body size and, to some extent, phylogeny, and suggest that there has been an evolutionary response to selection for low-frequency songs by birds in low-forest habitats. Our analysis of ambient noise in low-forest and grassland habitats suggests that this selection might result in part from a relatively quiet region of the ambient noise spectrum in low-forest habitat because of the comparative lack of wind-generated noise and of the abundance of insect-generated sound.