The role of unshared songs in singing interactions between neighbouring song sparrows

In this paper we focus on the potential advantage song repertoires may provide in singing interactions between two birds. We have previously shown that neighbouring song sparrows, Melospiza melodia, in our population countersing with shared songs: a bird escalates an interaction by replying with the same song type his neighbour just sang ('type matching'), or sends a directed but less threatening signal by replying with a different but still shared song ('repertoire matching'). In the present study we tested and confirmed two predictions: (1) that a bird would be more likely to perceive a neighbour's song as directed at him if it was shared than if it was unshared; (2) that an unshared song would be a more effective de-escalation signal than a shared song. In the first of two playback experiments, subjects responded with shorter latencies to neighbours' shared songs than to their unshared songs. In the second, 'interactive' playback experiment, a playback trial began when the subject sang a shared song type. We replied with the neighbour's matching type (an escalation signal), until the subject approached, and then switched to either a different shared song (a 'repertoire match') or to an unshared song. As predicted, subjects responded less aggressively and departed sooner when the switch was to an unshared song than when it was to a shared song.