The evolution of bird song: male and female response to song innovation in swamp sparrows

Abstract

Closely related species of songbirds often show large differences in song syntax, suggesting that major innovations in syntax must sometimes arise and spread. Here we examine the response of male and female swamp sparrows,Melospiza georgiana , to an innovation in song syntax produced by males of this species. Young male swamp sparrows that have been exposed to tutor songs with experimentally increased trill rates reproduce these songs with periodic silent gaps (Podos 1996, Animal Behaviour,51, 1061–1070). This novel temporal pattern, termed ‘broken syntax’, has been demonstrated to transmit across generations (Podos et al. 1999, Animal Behaviour,58, 93–103). We show here that adult male swamp sparrows respond more strongly in territorial playback tests to songs with broken syntax than to heterospecific songs, and equally strongly to conspecific songs with normal and broken syntax. In tests using the solicitation display assay, adult female swamp sparrows respond more to broken syntax than to heterospecific songs, although they respond significantly less to conspecific songs with broken syntax than to those with normal syntax. We conclude that sexual selection by female choice is in this case conservative, acting against the spread of innovation. The conservative effect of female preferences must sometimes be overcome, however, to allow the evolution of the major structural differences in song observed between species.