Within-song complexity in a songbird is meaningful to both male and female receivers

Abstract

Empirical studies on song ‘complexity’ have mainly focused on the effect of repertoire size. Whether differences in song complexity that are not correlated with longer signals or larger repertoires might also be meaningful for either same-sex competitors or potential mates has rarely been addressed. Male chaffinches, Fringilla coelebs, have small repertoires, with typically two or three different song types, but the number of different trill phrases within songs varies considerably. To test whether this variation is meaningful to receivers, we compared the responses of both male and female receivers to songs of equal duration but varying in the number of trill phrases. Males were tested in the field with playback experiments and females with an operant preference test in the laboratory, with song as a reinforcer. Both males and females gave stronger responses to songs with more phrases in the trill. In the literature, females are normally seen as driving songs towards more complexity. However, our results suggest that both intra- and intersexual selection can contribute to the evolution of more complex songs, but also that the salience of different song types within a male's repertoire varies.