The receiver-dependent cost of soft song: a signal of aggressive intent in songbirds

► In several songbird species soft song is an aggressive signal that predicts attack. ► We asked if the reliability of soft song is enforced by a social cost. ► We measured aggressive response of male song sparrows to soft and broadcast song. ► Subjects varied in aggressiveness but most responded more strongly to soft song. ► Retaliation against soft song is a cost that could explain the signal's reliability.