Neighbour-stranger call discrimination in a nocturnal rail species, the Corncrake Crex crex

The acoustic signals of birds are commonly used for individual recognition. Calls or songs allow discrimination between parent and offspring, between mates and between territorial neighbours and strangers. In this study, we investigated vocal neighbour-stranger discrimination in a nocturnally calling rail species, the Corncrake, Crex crex. We conducted interactive playback experiments with 43 males. All males were tested twice on the same night, and their responses to the calls of a familiar neighbour and the calls of an unfamiliar stranger were measured. The Corncrake males responded more aggressively to the playback of a stranger's calls. They approached the speaker more rapidly, spent more time close to the speaker and physically attacked the speaker more frequently. We found no significant differences between the vocal responses to the playback of neighbours' and strangers' calls. Thus, although calling plays an important role in Corncrake territorial interactions, it is only a first line of defence in which males signal aggression towards intruders by calling, similar to singing in passerines. The lack of differences in vocal responses and the presence of clear differences in other behavioural responses demonstrate that the absence of a differential vocal response does not imply the absence of discrimination. Although the individual nature of the Corncrake call has been mentioned in a few previous studies, this study provides the first experimental evidence that Corncrake males indeed use calls for neighbour-stranger discrimination. Notably, because of the Corncrake's dense wet meadow habitat and its tendency to signal at night, it is probable that acoustic individual discrimination in the Corncrake is crucial for making correct decisions during aggressive encounters with rivals.