The communicative significance of song frequency and song length in territorial chiffchaffs

Spectral and temporal measures of vocalizations have been found to correlate with physical parameters that affect the fighting ability in various species of frogs, mammals, and birds. This correlation could play a role in communication for intersexual mate attraction, as well as intrasexual competition. In this study, we investigated whether the frequency and length of territorial songs reflects the body size (tarsus length, weight) of males in a small passerine species, the chiffchaff Phylloscopus collybita. We found a tendency for a negative correlation between song frequency and tarsus length, but no significant links with song length. Furthermore, we carried out playback experiments to see whether the variation in song frequency and length is meaningful to receiving males and found that both parameters affected the males’ responses. We found that the response strength was related to the frequency difference between the songs of the tested male and the playback stimulus, rather than only to the stimulus frequency manipulation (increased/decreased frequency). Males showed a mixed strategy of moderate attack response, combined with a low approach response, when relatively low-frequency song stimuli were played back. In contrast, manipulation of the stimulus song length (shortened/elongated) was a better predictor of response strength than the relative differences in song length. Males attacked more and approached closer in response to elongated stimuli. Our data show how multiple messages can be encoded in different signal components; song frequency most likely conveys information about some aspect of fighting ability, whereas song length probably signals motivation to escalate into a fight among male chiffchaffs.