Acoustic and visual traits predict species nuclearity in Neotropical mixed-species bird flocks.

Mixed-species bird flocks constitute a striking example of heterospecific sociality, offering participants potential benefits such as enhanced foraging efficiency and reduced predation risk. Yet, not all species join these flocks with the same frequency. Moreover, beyond simply participating, species may play distinct roles within flock networks. While some bird species may exhibit high nuclearity and occupy a central position in flock formation, cohesion, and dynamics (i.e., social hubs), others are merely peripheral followers. Understanding how phenotypic and ecological traits relate to both flocking propensity and network position can provide key insights into the drivers of interspecific social behavior in birds. Using a comprehensive dataset of over 3,000 flocks, we applied network theory to explore the relationship between species-level network metrics (connectivity, strength, and closeness), flocking propensity, and key phenotypic traits (residual eye size, beak shape, maximum song frequency, and plumage coloration) in Neotropical birds. Bayesian phylogenetic models revealed that flocking propensity was positively associated with maximum song frequency and carotenoid-based coloration, and negatively associated with residual eye size. In addition, the extent of white in plumage significantly predicted species centrality. This is the first study to evaluate whether acoustic and visual traits predict both flocking propensity and a species' structural role within flock networks. These findings, along with the moderate phylogenetic signal detected in network metrics, suggest that the evolution of heterospecific sociality may involve distinct morphological and sensory adaptations.