Individually Specific Call Feature Is Not Used to Neighbour-Stranger Discrimination: The Corncrake Case

In various contexts, animals rely on acoustic signals to differentiate between conspecifics. Currently, studies examining vocal signatures use two main approaches. In the first approach, researchers search for acoustic characteristics that have the potential to be individual specific. This approach yields information on variation in signal parameters both within and between individuals and generates practical tools that can be used in population monitoring. In the second approach, playback experiments with natural calls are conducted to discern whether animals are capable of discriminating among the vocal signatures of different individuals. However, both approaches do not reveal the exact signal characteristics that are being used in the discrimination process. In this study, we tested whether an individual-specific call characteristic--namely the length of the intervals between successive maximal amplitude peaks within syllables (PPD)--is crucial in neighbour-stranger discrimination by males of the nocturnal and highly secretive bird species, the corncrake (Crex crex). We conducted paired playback experiments in which corncrakes (n = 47) were exposed to artificial calls with PPD characteristics of neighbour and stranger birds. These artificial calls differed only in PPD structure. The calls were broadcast from a speaker, and we recorded the birds' behavioural responses. Although corncrakes have previously been experimentally shown to discriminate between neighbours and strangers, we found no difference in the responses to the artificial calls representing neighbours versus strangers. This finding demonstrates that even if vocal signatures are individual specific within a species, it does not automatically mean that said signatures are being crucial in discrimination among individuals. At the same time, the birds' aggressive responses to the artificial calls indicated that the information transmitted by PPDs is important in species-specific call recognition and may be used by males and/or females to evaluate sender quality, similarly like sound frequency in some insect species.