Nonpasserine bird produces soft calls and pays retaliation cost

Low-amplitude vocalizations produced during aggressive encounters, courtship, or both (quiet/soft songs) have been described for many species of song-learning passerines; however, such signals have not been studied among nonlearning birds. During aggressive interactions, apart from using the broadcast call, male corncrakes (Crex crex) produce a low-amplitude, gurgling-mewing call, which appears to be equivalent to soft songs of songbirds. Previous studies have shown that low-amplitude vocalizations are reliable signals of aggressive motivation. It is unclear, however, how the reliability of such signals is maintained. We experimentally tested whether males that use soft calls are more likely to attack later on and whether males respond differently to natural calls consisting of both broadcast and soft elements and to their modified versions where the soft elements were removed. Senders were more likely to attack the speaker if they earlier produced soft calls. Receivers were more likely to attack the speaker or retreat the speaker if the playback included soft calls. These results show that soft call is a signal of aggressive motivation in the corncrake, and we argue that the reliability of this signal is maintained by a receiver-retaliation rule. Key words: aggressive signal, Crex crex, retaliation rule, soft call, soft song. [Behav Ecol 22: 657-662 (2011)]