Is song-type matching a conventional signal of aggressive intentions?

Song-type matching is a singing strategy found in some oscine songbirds wit h repertoires of song types and at least partial sharing of song types betw een males. Males reply to the song of a rival male by subsequently singing the same song type. For type matching to serve as an effective long-distanc e threat signal, it must be backed up by some probability of aggressive app roach and impose some type of cost on senders that minimizes the temptation to bluff. Western subspecies of the song sparrow exhibit moderate levels o f song-type sharing between adjacent males and sometimes type match in resp onse to playback of song types they possess in their repertoires. Interacti ve playback experiments were used in order to examine the subsequent behavi our of type-matching birds and to quantify the responses of focal birds to type-matching versus non-matching stimuli. Birds that chose to type match t he playback of a shared song type subsequently approached the speaker much more aggressively than birds that did not type match. Moreover, birds appro ached a type-matching stimulus much more aggressively than a non-matching s timulus. These results and consideration of alternatives suggest that type matching in song sparrows is a conventional signal in which honesty is main tained by a receiver retaliation cost against bluffers.