Neighbors affect vocal behavior of tropical wrens: a multi-speaker density-manipulation experiment

For territorial animals, the behavior of conspecific neighbors sets the social context of communication. Despite numerous investigations of vocalizations related to territory defense and mate attraction, the effect of neighbor density on animal vocal behavior has received little attention, particularly in tropical animals and animals where both sexes produce complex acoustic signals. In this study, we used an innovative multi-speaker playback experiment to manipulate the apparent density of neighbors in Rufous-and-white Wrens, Thryophilus rufalbus, living in Costa Rica’s tropical dry forest. In this tropical songbird, both males and females, defend year-round territories and sing complex, learned songs for territory defense. We recorded the singing behavior of 24 subjects (12 pairs) and then we used an array of six loudspeakers to simulate the presence of six new territorial neighbors (3 simulated pairs) outside each subject pair’s breeding territory. The stimuli persisted for three consecutive days with both male and female songs broadcast at a natural rate from dawn to dusk. We found that the mean male song rate increased by almost fifty percent in response to the simulated increase in local density. Females showed less frequent song-type switching rates following the simulated increase in local density, although it was a marginal increase. These findings reveal that male and female songbirds’ vocal behavior varies with the local density of territorial neighbors. We conclude that birds are sensitive to acoustic signals of conspecific density arising from sounds beyond their territory boundaries and that they use this public information to guide their vocal behavior.

Methods
Playback design: we developed a six-speaker playback design, consisting of three stereo pairs of customized digital loudspeakers to simulate three new neighboring pairs. Details of the custom-built loudspeakers are provided by Mennill et al. (2018). We recorded the vocal responses during 1 hour of the focal pair for three consecutive days. We compared the vocal responses of day 0 (pre-playback recording) vs. day 3. We evaluated three vocal responses: independent song, duet rate, and song-type switching rate in both, males and females Rufous-and-white Wrens.

Statistical Analysis: We performed a Linear Mixed Model to test the effect of higher perceived density of conspecific neighbors on Rufous-and-white Wren's vocal behaviors.