Dawn song in superb fairy-wrens: a bird that seeks extrapair copulations during the dawn chorus

摘要

Functional explanations of the dawn chorus in birds remain elusive. One hypothesis suggests that this acoustic display may play a role in female choice of extrapair males. Most young in cooperatively breeding superb fairy-wrens, Malurus cyaneus, are sired by extra-group males. Females initiate extra-group copulations exclusively through predawn forays to males singing in the dawn chorus. We measured variation between males in dawn singing at three levels of song production: song components, structured into songs, which make up recitals. We related this variation to independent measures of male quality and social status. Males sing two distinct categories of songs during the dawn chorus: a complex and variable chatter song and a more repeatable trill song. Dominant males with male subordinate helpers produced chatter songs at a greater rate than either dominant males without helpers or subordinates, suggesting a role in the competition between male group members. However, the trill song is implicated in female choice because older males sing songs with a longer trill component and have greater extrapair success, and tradeoffs between phrases within the trill component imply constraints on the length of the trill component in entirety that could enforce honesty. The dawn chorus of the superb fairy-wren may thus have a duel role, involving enforcement of dominance among male group members (male-male competition), and signalling attractiveness to mates (female choice).