Female calling? Song responses to conspecific call playbacks in nightingales, Luscinia megarhynchos

A crucial issue for understanding the evolution, functions and mechanisms of complex communicative signals such as birdsong is to disentangle signal structures that serve to convey information in different contexts. The two main singing contexts of European male songbirds are the attraction of a female and the defence of a territory from other males. The method of choice to experimentally investigate the use of song in male–male interactions is to conduct playback studies, in which song is played to a resident male. Responses to the presence of a female, however, are hardly testable this way, since females of most European bird species do not sing. However, females do call in several situations. In a playback experiment, we investigated whether free-ranging male nightingales change their nocturnal singing in response to playbacks consisting of calls produced by either female or male conspecifics. In both cases, nightingales changed their singing style as compared to their singing before playback, and these changes were different in response to male and female calls. Males sang fewer whistle songs after male call playbacks and started to produce ‘initial whistles’ earlier when hearing female calls. Male call playbacks also led to an increase in song duration. An analysis of the call characteristics of both sexes used in the playbacks uncovered acoustic parameters that may account for the differentiated response. We conclude that male nightingales distinguish between female and male calls, and discuss the song characteristics that might be used to specifically address either males or females. Call playbacks proved to be useful for disentangling song characteristics that serve the multiple functions of birdsong.