The effect of social structure on vocal flexibility in monk parakeets

The social complexity hypothesis argues that communicative complexity arises as a result of social complexity, with this occurring through several potential mechanisms including plasticity and selection. Most research to date has focused on ultimate drivers of repertoire size, for example finding that cooperative breeding species exhibit larger repertoires. Until this date no study has focused on individual-level drivers of vocal diversity. Here, we examine social networks and vocalisations in wild colonial-nesting monk parakeets ( Myiopsitta monachus ). First, we recorded foraging, tolerance and aggression networks for 337 individuals over two years, as well as co-nesting and relatedness, and matched these with 5599 vocalisations recorded for 229 individuals over this same period. Overall, we found that all individuals exhibited high contact-call diversity; however, individual-level diversity increased with age and with number of nest-mates. Call similarity was not predicted by relatedness, but individuals with stronger affiliative bonds had more dissimilar calls, suggesting an active process to sound unique amongst close associates. Finally, as predicted by the social complexity hypothesis, individuals living in larger groups had more diverse repertoires. Altogether, our results demonstrate a multi-faceted social influence on call content, diversity and repertoire size, exhibiting how fine-scale variation in social structure can influence expressed vocal complexity.