Ecological communities are comprised of species that interact with each other and those interactions ultimately generate community structure. Network theory provides a useful framework to study communities, by simultaneously considering species composition and the interactions among species. In this study, I use mixed-species flocks as model systems to gain insights on community and network structure. Specifically, I use co-occurrence network analyses to explore if avian mixed-species flocks change in richness and composition and/or in network structure and pair-wise associations, across elevations in the tropical Andes of Bolivia. Networks of flocking species changed both in composition and in the frequency and realization of pair-wise interactions across elevations, but changes in pair-wise associations explained most of network turnover along elevation. Pair-wise interactions changed rapidly, with shared species changing in position and importance within the network. Network dissimilarity was mostly explained by changes in the nature of associations rather than by differences in composition. Altogether, results show that montane mixed-species flocks are composed of loosely connected species and that most species have the potential of switching associations, often increasing in association strength at high elevations (up to 3150m). Networks increased in connectivity and cohesion with elevation; flocks in lower elevations had more connections and these were less even. Above 3150 m asl, there was rapid decay suggesting that flocks above this critical point are less connected and cohesive. This study exemplifies how combining community, network and pair-wise analyses can provide a more holistic view on the responses of species and assemblages to environmental gradients.

Flocks were surveyed between May and November 2016 and May and August 2017, along an elevational gradient (~1350 – 3500 m asl) in Cotapata National Park, a protected area in the Andes of western Bolivia. A flock was defined as a gathering of at least two species foraging < 10 meters from each other while moving in the same direction.Flock data were collected with the “gambit of the group” method, where several individuals observed at the same time and place are assumed to be associated.

Raw data on presence-absence of 164 bird species participating of 358 mixed-species flocks in the Andes of western Bolivia are included in the dataset, with species in columns and flocks in rows 

Species are presented in columns and flocks in rows.

Column 1: Flock, includes the flock ID number

Columns 2 – 165: Species, nomenclature coincides with that of Jetz et al. (2012) available in  https://birdtree.org/

Column 166: Elevation band, range of elevation in m asl, where the flock was observed.

Column 167: Site, reference elevation (in m asl) used in the manuscript to describe the elevational position of the network.