Pairs of duetting birds can sing coordinated duets with such precision that they are often mistaken for a single individual, yet little is known about how this impressive temporal synchronization is achieved. We experimentally examined duet coordination in male happy wrens, held briefly in captivity, by playing song phrases from their partner at different distances and tempos. Males were more likely to respond to songs played nearby, but did not vary their amplitude to compensate for their partner's simulated distance. Males modified their song rate to match the manipulated female playback tempo, indicating that they listen and respond to each female utterance. Each happy wren has a sex-specific repertoire of about 40 different song phrases and pairs combine particular phrases according to pair-specific duet ‘codes’, creating a further challenge for coordinating duets. We found that most males produced the appropriate phrase to reply to the female playback song in the absence of any other potential cues, sometimes delivering the correct song phrase type within 0.5 s of the start of the very first female playback heard. These experiments demonstrate rapid decision making and vocal production, indicative of sophisticated underlying cognitive processing, and provide a novel experimental technique to investigate the mechanisms controlling vocal duets.