Abstract
Although it is widely accepted that male songbirds sing to repel males or to attract or stimulate females, it is not well understood why the structure of bird song is so complex. In most songbirds, individuals sing different versions of the species-specific song (i.e. song or syllable types) to form a song repertoire. The composition of repertoires (the song types that constitute the repertoire), the size of repertoires (number of song types), the rate of song type switching and the way single song types are performed (song rate, percentage performance time, song duration), all differ considerably within and among species (reviews in Kroodsma and Miller 1982; Searcy and Andersson 1986; Kroodsma and Byers 1991).