Riverside wren pairs jointly defend their territories against simulated intruders

Duets are a jointly produced signal where two or more individuals coordinate their vocalizations by overlapping or alternating their songs. Duets are used in a wide array of contexts within partnerships, ranging from territory defence to pair bond maintenance. It has been proposed that pairs that coordinate their songs might also better coordinate other activities, including nest building, parental care and defending shared resources. Here, we tested in the riverside wren (Cantorchilus semibadius), a neotropical duetting species that produces highly coordinated duet songs, whether males and females show similar responses to playback. During territorial disputes in songbird species, individuals tend to direct their attention towards same-sex territorial intruders, but this bias might be less pronounced in duetting species. We performed a dual-speaker playback experiment to examine how mated individuals respond to speakers broadcasting female-versus-male duet contributions. We found that riverside wrens have high levels of converging behaviour by duetting and remaining in close proximity to one another when responding to simulated paired intruders. Males and females spent more than 80% of their time less than 1 m apart while defending their territory. Both individuals in a pair aggressively engaged with both male and female simulated trespassers by approaching equally close and spending equal time near the two speakers. These results suggest that both sexes perceive a paired territorial intrusion as a similar threat and that both partners are highly invested in defending the shared resources. This study is one of the few to demonstrate equal attention and aggression from mated pairs towards simulated same-sex and opposite-sex intruders. We suggest that pairs responding together, in close proximity to one another, might be favourable in duetting species when defending the territory because maintaining a close distance between partners facilitates the extreme coordination of their joint territorial signals.