Behavioural consequences of conspecific neighbours: a systematic literature review of the effects of local density on avian vocal communication

Among territorial birds, the number of conspecific neighbours sets the social context of communication. There have been many investigations of vocal behaviour and its important role in territory defense and mate attraction in birds; however, the effect of the density of conspecific neighbours on avian communication has received little attention. In this study, we reviewed the literature on the influence of local density on vocal communication in birds, exploring how the number of conspecific neighbours influences avian vocal behaviour, by conducting a literature review following the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) in two databases: Web of Science and Scopus. We also included additional articles from a more flexible search in Google Scholar. We found 19 studies of birds that included data on vocal behaviour in relation to the number of conspecific neighbours. The majority of these studies (14 of 19) showed that local density influences avian vocal behaviour. Overall, these studies revealed an effect of local density of conspecifics on song rate (expressed as song rate, syllable rate, solo song rate, or singing rate). Other vocal behaviours that were shown to vary with local density included song-type switching, song duration, peak frequency, and song variation. Most studies focused on temperate-zone species where males are the predominant singers. Our results reveal a bias towards the study of male songs in the context of local density, with very few studies on female song; this corresponds with a historical geographical focus on temperate birds. We present experimental design suggestions for future investigations, including predictions for males and females of tropical and temperate species. Finally, we argue that more research on this topic is needed, and that investigations of vocal communication will benefit from further study of the influence of density on avian vocal behaviour.