Multi-level analysis of monk parakeet vocalisations shows emergent dialects between cities in the European invasive range

Geographic differences in vocal dialects have provided strong evidence for animal culture, with patterns likely arising from generations of social learning and transmission. The current knowledge on the evolution of vocal dialects has predominantly focused on fixed repertoire, territorial song. The study of vocal dialects in open-ended flexible learners and in contexts where vocalisations serve other functions is therefore necessary for a more comprehensive understanding of vocal dialect evolution. Parrots are open-ended vocal learners where vocalisations are used for social contact and coordination. Here, we recorded monk parakeets (Myiopsitta monachus) across multiple spatial and social scales in their European invasive range. We then compared calls using a multi-level Bayesian model and sensitivity analysis, with this novel approach allowing us to explicitly compare dialects at multiple spatial scales, even in unmarked populations. We found support for founder effects and/or cultural drift at the city level, consistent with passive cultural processes leading to large scale dialect differences. We did not find a strong signal for dialect differences between groups within cities, suggesting groups did not actively converge on a group level signal, as expected under the group membership hypothesis. We demonstrate how our sensitivity analysis highlights the robustness of the results and offer an explanation that unifies the results of prior monk parakeet dialect studies.