Network structure of avian mixed-species flocks decays with elevation and latitude across the Andes

Abstract

Birds in mixed-species flocks benefit from greater foraging efficiency and reduced predation, but also face costs related to competition and activity matching. Because this cost-benefit trade-off is context-dependent (e.g., abiotic conditions, habitat quality), the structure of flocks is expected to vary along elevational, latitudinal, and disturbance gradients. Specifically, we predicted that the connectivity and cohesion of flocking networks would (1) decline towards tropical latitudes and lower elevations, where competition and activity matching costs are higher, and (2) increase with lower forest cover and greater human disturbance. We analysed the structure of 84 flock networks across the Andes, and assessed the effect of elevation, latitude, forest cover and human disturbance on network characteristics. We found that Andean flocks are overall open-membership systems (unstructured), though the extent of network structure varied across gradients. Elevation was the main predictor of structure, with more connected and less modular flocks upslope. As expected, flocks in areas with higher forest cover were less cohesive, with better defined flock subtypes. Flocks also varied across latitude and disturbance gradients as predicted, but effect sizes were small. Our findings indicate that the unstructured nature of Andean flocks might arise as a strategy to cope with harsh environmental conditions.