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.

Methods
Avian mixed-species flocks were surveyed for two consecutive years, from May to October 2016 and May to August 2017, along a continuous elevational gradient (~1,350 – 3,550 m.a.s.l.) in Cotapata National Park, a protected area in the Andes of western Bolivia. The gradient was sampled 16 times (8 times per year) through randomized 350-m elevation surveys, so that elevations were not sampled sequentially to avoid temporal biases. Each survey consisted of walking at a slow constant pace along the transect and, for each flock encountered, recording GPS coordinates, elevation, and the number and identity of all participating species. we defined a flock as a gathering of at least two species foraging < 10 meters apart while moving in the same direction. Flock data were collected using the “gambit of the group” method, where several individuals observed at the same time and place are assumed to be associating. To diminish the bias of registering non-flocking individuals that increased their foraging activity while the flock was present, we recorded the whole encounter until the flock was unreachable and excluded all species that remained in the study areas (feeding, perching and/or vocalizing) after the flock had left. In our study, each detected flock represents a snapshot of the realized associations resulting from individual birds deciding whether to join the flock and, thus, flocks observed on different days were considered independent replicates of flock composition and richness. Species richness is known to be correlated with observation time; thus, we further standardized our effort by including only flocks that were observed for between 10 and 20 minutes. Data were organized into abundance matrices, with flocks as columns and species as rows.