Abstract and Summary

I present results in support of the hypothesis that song repertoires of male varied thrushes are organized to reduce stimulus generalization by other birds. The songs are simple tones, modulated in amplitude. Described along two dimensions, dominant frequency (Hz) and period of modulation (ms), the songs within repertoires are less similar than would be expected if they were drawn at random from all the songs in the population. This may enhance the effectiveness of repertoires in repelling other males and in attracting and stimulating females.