Temporal coordination signals coalition quality

Coordinated displays are widely used to defend shared resources, and may signal coalition strength so that groups can assess the relative competitive ability of rivals and avoid unnecessary fights. Joint vocal displays are known to facilitate numerical assessment of relative group size in some animals, but it is not known whether features of coordinated vocal displays can signal coalition strength independent of coalition size. We show experimentally that precise coordination between partners in avian duets increases the perceived threat of these joint territorial displays, and provide the first evidence that established pairs produce more highly coordinated duets than new pairs. Duet precision thus serves as an honest signal of coalition strength. This is the first reported evidence of cooperative benefits for precise temporal coordination of signals.