Duet structure and the adaptive significance of coordinated singing in the black-bellied wren

Abstract

In a diverse array of bird species, mated pairs coordinate their songs to form vocal duets. I investigated duet structure and the adaptive significance of duet participation in free-living black-bellied wrens (Thryothorus fasciatoventris). Both sexes sign repertoires of sexually diphonic songs which they combine to create vocal duets. Pairs duet at elevated levels during border disputes, territory intrusions, territory expansion, mate switching, and nest building, and in response to duet song from conspecifics and buff-breasted wrens (T. leucotis). I used a series of playback experiments to test the functions of duet participation during agonistic encounters, and to examine the roles of the sexes in territory defense. Males initiated more songs during opposite-sex playback than during same-sex playback and both sexes were more likely to answer their mate's songs when the mate was physically closer. These patterns support the hypothesis that duetting during territorial encounters allows males to identify and locate their mates. Males with dependent juveniles were found to defend inter-sexually, but other birds exhibited a same-sex bias in territory defense. This result is consistent with a model that assumes birds act to maximize their own fitness. Unique 'duet codes' that link the male song-type that a female hears to the song type that she sings in response could provide a mechanism for mate recognition through duet song. I tested for duet codes by inducing females to answer playback of male songs. I found that females abided strictly by duet codes. Different females use different codes, which do not change over time or after re-mating. There is some evidence that codes are learned. Throughout this study, I recorded and played back sounds with MiniDisc (MD) digital audio recorders. I compared the performance of MD recorders with that of professional grade cassette recorders and found that MD recordings exceed cassette recordings in several measures of found fidelity. I also found that MD recorders introduce acoustic artifact in certain recordings. Nevertheless, these units are appropriate for most field recording and playback applications, and afford numerous advantages over audio cassette recorders.