Resources and seasonality drive the composition of mixed-species bird flocks along an elevational gradient in the Himalaya

Mixed-species bird flocks are cohesive networks of interacting insectivorous bird species that benefit from reduced predation risk and/or enhanced resource access. We studied how species' propensity to participate in mixed flocks, and the number and strength of interspecific associations within flocks, changed along elevational (low, mid, high) and seasonal (winter, spring) gradients in the western Himalaya. We expected species to have high propensities, and greater number and strength of interspecific associations when resources are scarce, and to decline when resources are abundant. We first used species occurrence within and outside mixed flocks to calculate species-specific flocking propensity and then performed network analyses to quantify the proportion of realized interspecific associations (network density) and their strength (weighted degree), using an abundance-based null model to control flocking species' availability. Further, we quantified arthropod prey availability in winter by using a branch bagging technique. Insect availability and arthropod diversity decreased with increasing elevation in winter and species’ flocking propensities, network densities and weighted degrees increased with elevation, possibly to benefit from facilitative interactions that increase foraging success. In spring, as more resources become available and/or bird start investing more time in breeding, flocking propensities, network densities and species’ weighted degrees declined at all elevations when compared with winter. During resource-scarce periods, species might find solitary foraging prohibitive because of reduced resource access and costs of vigilance therefore leading to the formation of networks of facilitative associations.