Does twitter song amplitude signal male arousal in redwings (Turdus iliacus)?

Abstract

Bird songs may vary in amplitude for several reasons. Variations due to differences in environmental conditions are well known but whether signal information varies with song amplitude is less well known. In some species quiet songs are heard as a soft twitter. These twitter songs are common in Turdus species and may be used during escalated close range encounters when a quiet song will attract less attention from others. Male redwings (T. iliacus) sing a terminating twitter part that is quieter and highly variable both between and within males compared with the introductory motif part. The twitter song of redwings, however, is often louder than the twitter in other Turdus species, especially during escalated song encounters. The seasonal variation in twitter duration also suggests that the twitter may signal increased aggression. We tested how male redwings responded to an assumed escalation in signalling. In an interactive playback experiment we increased the amplitude of the twitter songs we played back in response to an increase in twitter duration by the subject. Males gave stronger responses to louder twitter. This suggests that twitter amplitude may signal arousal in male redwings.