Social environment affects vocal individuality in a non-learning species.

Individual recognition is fundamental to the social behaviour of many animals. In the context of territorial behaviour, animals in high-density populations encounter conspecific rivals and potential mates more frequently, which should enhance the individuality of territorial signals to facilitate recognition among conspecifics. We investigated vocal individuality in male territorial calls of two populations of little owls (Athene noctua) with different densities. Further, to explore the potential influence of local population distribution on individuality, we also examined isolated males without neighbours and clumped males with neighbours. Our findings indicate higher individuality at higher densities across both scenarios, measured using two individuality metrics: Beecher's information statistic and Discrimination score. Clumped males exhibited significantly lower acoustic niche overlaps (i.e. higher vocal individuality) compared to isolated males. However, only a non-significant trend for lower acoustic niche overlaps (i.e. higher vocal individuality) was found for males from high density compared to low density populations. This suggests that the immediate social environment might be more influential than larger-scale population density patterns. This study suggests that vocal individuality in a territorial species is influenced by conspecific density, similar to findings in group-living and colonial species.