Kyle Schuyler Van Houtan, Jose-Ignacio Rojas-Moscoso, Hope Noelle Van Houtan, Oscar Gonzalez

Parrots are a highly intelligent taxon whose complex behaviors in wild societies require description. Here we observed 12 species of parrots, macaws, and parakeets in mixed flocks foraging on exposed cliffs in southeast Perú. For each species, we developed a single bootstrapped index of sociality from 9 derived metrics of abundance, chronology, functional roles, and agonistic interactions. This multivariate index emphasizes species that join flocks in large numbers, participate early, serve in functional roles, interact with others, and are socially dominant. We used Random Forest (“RF”) algorithms to build nonlinear multiple regressions to assess and rank the influence of a suite of taxonomic and morphometric factors on sociality. The RF models performed well (R2 = 0.96) and indicate species with smaller brains (controlled for body size) and lower dispersal abilities are most social, though the underlying mechanisms may be indirect. In addition, we document a distinct sequence of species flock participation where subordinate species serve as nuclear species that initiate flock assembly and foraging, while dominant species serve as sentinels, foraging after subordinates. This suggests cooperation in this mixed society features sequenced tradeoffs and reciprocal altruism.