Strong ungraded responses to playback of solos, duets and choruses in a cooperatively breeding Neotropical songbird

Coordinated vocal displays of cooperatively breeding animals provide a compelling model for investigating the opposing motivations for engaging in conflict versus cooperative behaviours. Hypotheses for the function of coordinated vocal displays differ with respect to these motivations and have been traditionally investigated by using playback to simulate varying degrees of threat to individuals and groups. We evaluated the function of coordinated vocal displays by presenting territorial groups of cooperatively breeding rufous-naped wrens, Campylorhynchus rufinucha, with three playback stimuli: solos, duets and choruses. We found that all groups responded strongly to playback by approaching the loudspeaker together, vocalizing, and performing visual displays. A composite playback response measure showed significantly more aggressive reactions to all playback treatments compared to a preplayback control period, yet did not vary across solo, duet and chorus treatments. This suggests that the playback stimuli represented equally strong threats despite the varying numbers of contributors to each stimulus, and does not support the hypothesis that coordinated vocalizations are graded signals of threat in this species. Our findings stand in contrast to previous playback studies that have reported an increase in aggression with an increasing number of simulated intruders, or an increase in coordinated vocalizations in response to solo playback. We interpret the results of our study as evidence that coordinated vocalizations function in the cooperative behaviour of joint territory defence in the rufous-naped wren.