DOI: 10.5061/dryad.mpg4f4qzd
title: Data from: Temperature and vegetation complexity structure mixed-species flocks along a gradient of elevation in the tropical Andes

description: 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.
description: 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 &lt;
 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.
description: Data is in an Excel file.  First sheet
 contains metadada Data is organized in the following
 two sheets: Flock_Taxo    Taxonomy and common names for
 species in 368 mixed-species flocks in the Bolivian Andes. Scientific
 taxonomy and English names follow the eBird/Clements checklist.
 Scientific name    <br> Order    <br> Family  
  <br> English name    <br> Mean abundance - Average number of
 individuals of the species observed per mixed-species flock<br> N
 flocks - Number of flocks in which the species was observed
 Flocks_Numb    Data used for analyses in "Temperature and
 vegetation complexity structure mixed-species flocks along a gradient of
 elevation in the tropical Andes" Flock - Flock
 number    <br> elev - Elevation in m asl    <br> temp - Mean
 daily temperature (collected in the field)<br> fruits - Fruit
 diversity (Shannon's diversity index H')<br> arthropods -
 Arthropod diversity (Shannon's diversity index H')<br>
 complexity - Vegatation complexity (modified version of the ‘understory
 height diversity index (UHD)’ used by MacArthur and MacArthur,
 1961)<br> Columns 7-168 - Raw counts of flocking species (one
 species per column). Species names correspond to the Scientific name
 column in Flock_Taxo.