Habitat associations of song characteristics in Phylloscopus and Hippolais warblers

Bird-song functions, such as mate attraction, species recognition, and territory defense, are closely linked to individual fitness. Thus, habitat characteristics that affect song transmission and degradation should exert a strong influence on the evolution of song attributes. Whereas different acoustic environments may influence the evolution of song characteristics, factors such as body size and evolutionary history of taxa are expected to constrain the amount of environmental variation in song properties. In the present study, we controlled for phylogeny and examined the effects of body mass and habitat structure on variation in song structure of 30 taxa of Phylloscopus and Hippolais warblers, which are a closely related group of birds that occupy a wide variety of habitats and show high variation in vocalizations. Habitat structure was strongly correlated with temporal characteristics of songs but not with most of the frequency-related attributes that we measured. Only the highest frequencies of songs varied with habitat structure. As predicted, species occupying closed habitats avoided the use of rapidly modulated signals and had song structures that minimized reverberation. Body mass covaried significantly with most of the song attributes. Smaller species used higher frequencies and had more notes in their songs compared with larger species.