Social costs are one mechanism whereby reliability in signalling systems can be maintained. We measured the strength of aggressive response to territorial playback to ask whether the reliability of 'soft song', a strongly aggressive signal in the song sparrow, Melospiza melodia, is enforced by a social cost in the form of the receiver's aggressive response. We also asked whether this cost is imposed by all receivers, or whether a differential response is found primarily or exclusively among the most aggressive subjects. We first measured the strength of each male's aggressive response to playback on his territory, and then tested his responses to 'warbled' soft song and to broadcast song playbacks. While we found substantial variation in individual aggressiveness, nearly all males responded more strongly to warbled soft songs. Thus we provide evidence that warbled soft song imposes a social cost in the form of receiver retaliation, and provide a possible explanation for how the signal's reliability is maintained. Questions about soft song remain, notably why selection should favour low amplitude in vocal signals of aggression in songbirds. © 2012 The Association for the Study of Animal Behaviour.