Canyon wrens alter their songs in response to territorial challenges

► We tested predictions of the motivation-structural hypothesis in canyon wrens. ► We also tested whether wrens use low-frequency sounds as signals of aggression. ► Territory-holding canyon wrens changed their songs in response to playback of conspecific song. ► They used lower song frequencies and harsh notes more often following simulated territorial intrusions. ► Ours is the first study to experimentally demonstrate facultative song adjustment in a passerine.