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

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 (~1350 – 3550 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 ~2300 m. The transition of
flocking behavior towards 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 (~2300 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.