Testing the function of song-matching in birds: Responses of eastern male song sparrows Melospiza melodia to partial song-matching

Song-matching has been hypothesized to be a signal of aggressive intentions whereby matching an opponent signals that the singer is likely to attack. Theory predicts that an aggressive signal should impose a cost that enforces the signal's reliability. A receiver-dependent cost imposed by the matched bird's aggressive retaliation has been proposed for song-matching. We tested for such a cost for partial song-matching in an eastern population of song sparrows where males lack the shared song types necessary for song type matching, but can perform partial song-matching using shared song segments. We tested aggressive response, as measured by average distance to a playback speaker, to partial-matching songs and non-matching songs. We predicted a stronger aggressive response to partial-matching songs, as has been shown for whole song-matching in western song sparrow populations. The birds in our study responded no differently to partial-matching and non-matching songs. Neither the distance to the playback speaker nor singing responses differed between playback treatments. Our results do not support a receiver-dependent cost to partial song-matching, as would be expected if partial-matching is a direct threat. Instead, we suggest that partial song-matching functions as a signal of attention.