You are what you tweet: vocal traits communicate threat in a duetting tropical bird

Animals may assess the vocal traits of opponents during resource competition to determine threat, which is a combination of the fighting ability and motivation of the opponent. Most studies have looked at how vocal traits reflect threat in males, but females and groups also vocalize during competition. Here, we examine how the vocal traits of males, females and male–female pairs signal threat in chestnut-backed antbirds, Poliocrania exsul, a tropical species that sings and duets to defend permanent territories. We quantified the threat of all birds in terms of both body size (indicative of fighting ability) and territory quality (indicative of fighting ability or motivation), measured as territory size and vegetation density, then recorded songs to analyse the vocal traits of each bird. We found that males and females communicated information about body size through the rate and fine-scale attributes of their songs and those communication strategies were largely similar between sexes. Furthermore, male–female pairs coordinated their songs into duets and the timing of their singing signalled the level of threat. Our results indicate that fine-scale attributes of vocalizations convey important information about individuals and groups in the context of competition.