The meaning of song repertoire size and song length to male whitethroats Sylvia communis.

The importance of intrasexual differences in repertoire size was investigated in a playback experiment with whitethroats, Sylvia communis. Fourteen unmated territorial males received three treatments: a small repertoire, a large repertoire, and a small-elongated repertoire where the last couple of elements had been repeated to give songs of about the length of large repertoire playback. Besides responding to the intruder's absolute repertoire size or song length, males might also take their own singing ability into consideration. Therefore, the playback was considered relative to the repertoire size and song length of the territory defending males. However, this seems not to have had an important effect on the response. There was a stronger response to large than to small repertoire playback and a stronger response to small-elongated than to small repertoire playback, whereas there was no difference between large and small-elongated repertoire playback. In general, the strongest response was given to playback of long song (large and small-elongated song repertoire), suggesting that males responded to song length rather than repertoire size in short-term interactions with an intruder. This agrees with the idea that song length indicates the degree of arousal of the singer. The lack of response to repertoire size suggests that the evolution of the complex song repertoire in whitethroats' motif song is not a result of intrasexual selection.