Song amplitude affects territorial aggression of male receivers in chaffinches

The evolution of bird song is closely related to sexual selection as birds use their songs for mate attraction and territorial defense. Recently, it has been shown that song amplitude can differ markedly between males and that females prefer louder songs. However, it is not known what constrains the production of loud songs. Here, we investigated how interindividual variation in song amplitude affects male--male territorial interactions. We simulated territorial intrusions by exposing male chaffinches (Fringilla coelebs) to song amplitudes consistent with the high and low end of the distribution of naturally occurring interindividual variation and found significantly stronger territorial behaviors in males exposed to high-amplitude songs. In particular, males sang less, approached the simulated rival closer, and stayed longer in the vicinity when the playback loudspeaker broadcasted louder songs. Our results demonstrate that the intensity of territorial behavior in chaffinches is modulated by the amplitude of songs to which they are exposed, suggesting that social aggression is likely to constrain song performance in this species. We conclude that the variation in song amplitude between males is, at least partly, a result of male--male competition as probably not all males are able to bear the potential costs of increased social aggression. Copyright 2011, Oxford University Press.