Temperature and vegetation complexity structure mixed-species flocks along a gradient of elevation in the tropical Andes

ABSTRACT Mixed-species flocks constitute community modules that can help test mechanisms driving changes to community composition across environmental gradients. Here, we examined elevational patterns of flock diversity (species richness, taxonomic diversity, species, and guild composition) and asked if these patterns were reflections of the full bird community at a given elevation (open-membership hypothesis), or if they were instead structured by environmental variables. We surveyed both the overall avian community and mixed-species flocks across an undisturbed elevational gradient (∼1,350–3,550 m) in the Bolivian Andes. We then tested for the role of temperature (a surrogate for abiotic stress), resource diversity (arthropods, fruits), and foraging niche diversity (vegetation vertical complexity) in structuring these patterns. Patterns for the overall and flocking communities were similar, supporting our open-membership hypothesis that Andean flocks represent dynamic, unstructured aggregations. Membership openness and the resulting flock composition, however, also varied with elevation in response to temperature and vegetation complexity. We found a mid-elevation peak in flock species richness, size, and Shannon's diversity at ∼2,300 m. The transition of flocking behavior toward a more open-membership system at this elevation may explain a similar peak in the proportion of insectivores joining flocks. At high elevations, increasing abiotic stress and decreasing fruit diversity led more generalist, gregarious tanagers (Thraupidae) to join flocks, resulting in larger yet more even flocks alongside a loss of vegetation structure. At lower elevations, flock species richness increased with greater vegetation complexity, but a greater diversity of foraging niches resulted in flocks that were more segregated into separate canopy and understory sub-types. This segregation likely results from increased costs of interspecific competition and activity matching (i.e., constraints on movement and foraging rate) for insectivores. Mid-elevation flocks (∼2,300 m) seemed, therefore, to benefit from both the open-membership composition of high-elevation flocks and the high vegetation complexity of mid- and low-elevation forests. LAY SUMMARY We investigated changes to the diversity and membership of flocks of birds across elevations in the Bolivian Andes, and the mechanisms driving these changes. Changes to flock membership largely reflected elevational changes to the whole bird community, suggesting Andean flocks have open membership, unlike the flocks of lowland Amazonia. Elevational changes to flock richness, size, and membership were mostly associated with effects of temperature (abiotic stress) and the diversity of vegetation layers within the forest. Temperature and fruit diversity were associated with the number of individuals of each species, with larger flocks at high elevations, composed of generalist, gregarious tanagers. Increasing numbers of vegetation strata were associated with higher insectivore richness and abundance in flocks, explaining flock characteristics at middle and low elevations.