Song behaviour and reproductive strategies in a duetting wren,
 Thryothorus nigricapillus
 : II. Playback experiments

Four series of song playbacks to paired and unpaired resident birds were used to test hypotheses proposed for the function of male song, female song and vocal duets in the bay wren, Thryothorus nigricapillus, in the Republic of Panama. One series of experiments served as a control and established that bay wrens distinguished the duets of conspecifics from those of heterospecifics; a second series indicated that these results were not dependent on which type of male vocalization was used as a stimulus. Two series of experiments compared the responses of paired and unpaired males and females to playbacks of male versus female song. Both paired and unpaired females responded strongly to playback of female song and weakly to playback of male song. These data are consistent with previous results that showed no effect of mating status on female song rates and support the hypothesis that female song functions in intra-sexual aggression, probably in the context of territorial defence. Furthermore, because females initiate all duets in this species, females may be singing independently of males, and thus it is only male behaviour that produces a duet. In contrast to what was observed in females, males responded strongly to playback of both male and female song, but paired males responded more strongly to playback of male song, and unpaired males responded more strongly to playback of female song. These results provide a preliminary indication that female songs are attractive to males and that males may participate in duets as a means of acoustically guarding their mates from other males. 1996 The Association for the Study of Animal Behaviour Most of the work on the structure and function of avian song has been conducted on temperate zone species in which typically only males sing. It is not yet understood why, in many species of tropical birds, both males and females sing, and why, in over 200 species, these songs occur as vocal duets (Thorpe 1972; Kunkel 1974; Farabaugh 1982). Originally, it was hypothesized that vocal duetting was the result of similar selective pressures on males and females, functioning in pair bond maintenance and/or joint territory defence (e.g. Armstrong 1963; Thorpe 1963, 1972). More recently, it has been suggested that vocal duetting might also function in acoustic mate guarding (Brown & Lemon 1979; Sonnenschein & Reyer 1983), such that a bird vocally responds to its partner’s songs to signal that its partner is already paired and therefore unavailable as a potential mate. Under this hypothesis, duetting is viewed less as a cooperative effort between mates than as the result of independent male and female strategies. In a companion paper, I described how I used mate removal studies to test both the pair bond maintenance and territory defence hypotheses in the bay wren, Thryothorus nigricapillus, a tropical species in which females initiate precisely timed antiphonal duets with their mates (Levin 1996). Neither hypothesis was supported, and results suggested that male and female bay wrens might use their songs to pursue somewhat different reproductive strategies. One likely function of female song is intra-sexual aggression. Songs and calls appeared to be used by unpaired males to attract females; the function of song in paired males remains unclear. Correspondence and present address: R. N. Levin, Department of Biology, Pomona College, Claremont, CA 91711, U.S.A. (email: rlevin@pomona.edu). 0003–3472/96/121107+11 $25.00/0 ? 1996 The Association for the Study of Animal Behaviour