| Summary: | Understanding the influence of species interactions on community structure is a long-standing goal in ecology. While many studies have focused on negative biotic interactions, the role of other mechanisms has received less attention, in particular, facilitation. In birds, a striking case of facilitation occurs in mixed-species flocks, in which individuals of different species move and forage as a group to obtain benefits from the association. These associations of species in mixed flocks have been described in different habitats during the last century; however, there is still much debate regarding the prevalence of this foraging strategy and the role it plays in Neotropical bird communities. In this study, I integrated data from mixed species flocks observations and species occurrence to investigate how facilitative interactions influence the structure of Neotropical bird communities across a 3000-m elevational gradient on the eastern slope of the Andes in Peru. First, I examine how the structure of mixed flocks changes across elevations. Second, I quantify the stability of these multispecies groups over time. Third, I evaluate the association of several key habitat variables with flock diversity. Finally, utilizing a dataset for the entire forest bird community, I assess the prevalence and importance of mixed-species flocks across the gradient. The results showed that flocks were highly organized and stable across elevations. Flocks across the gradient exhibited a similar general structure, composed of a stable core group of species and a more dynamic component of attending species. This spatial and temporal analysis suggests that the stability of mixed-species flocks in the Andes is similar to what has been previously described in the Amazonian lowlands, with flocks exhibiting stable home ranges and core member composition over time. Vegetation structure explained 63% of variation in flock richness along the gradient, with number of trees and canopy height as primary predictors. Importantly, this study demonstrates that mixed-species flocks are used by more than a third of bird species present in the community, suggesting that these facilitative interactions are an important and underappreciated component of tropical bird communities. === Science, Faculty of === Zoology, Department of === Graduate
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