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.