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Duets are precisely coordinated acoustic displays, usually involving members of a mated pair. Studies investigating avian duet function often employ a single speaker playback system to compare response of focal pairs to simulated territorial intrusion by duetting and solo birds. It has recently been suggested that a 'stereo duet playback', in which male and female duet components are separated and broadcast through two different speakers, would provide a more realistic duet stimulus. We conduct the first comparison of a traditional single speaker versus a new stereo duet playback design and provide evidence that Australian magpie-larks, Grallina cyanoleuca, make significantly more flights towards duet playback presented in a more realistic stereo context. Male and female magpie-lark pairs did not split up and attack one 'intruder' each when presented with a stereo duet playback. Instead they moved towards the same speaker together as a united pair, showing a tendency to approach the speaker initiating the duet intrusion. Distance between the two speakers in a stereo duet design did not have a significant effect on the response variables measured. We conclude that magpie-larks can distinguish between use of a single speaker or stereo duet playback to broadcast a duet and suggest that cooperative defence against duetting intruders in magpie-larks is a result of joint territorial defence rather than intraspecific aggression against same-sex intruders. [ABSTRACT FROM AUTHOR]