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 and 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 (i) decline towards tropical latitudes and lower elevations, where competition and activity matching costs are higher, and (ii) 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. This article is part of the theme issue ‘Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes’.

Differences in mixed‐species bird flocks across forest succession: Combining network analysis and trait‐based ecology related to the fast‐slow continuum

“…As we have argued in earlier sections of Section 4, we see these patterns shaped by high resource and habitat availability allowing a speciose community of slower species(Huang et al, 2018;Schuldt et al, 2018), and increased vegetational complexity providing the opportunity for greater vertical strata partitionin, which may be linked to a tendency for more assortment by the birds. Given that other studies have proposed that elevational change produces similar shifts in MSFs(Montaño-Centellas, 2020;Montaño-Centellas et al, 2023;Shen et al, 2022), we suggest the vegetation complexity may be the underlying factor influencing both the elevational and disturbance gradients.…”