Species interactions with in mixed species bird flocks along an Elevational gradient in the Western Himalaya.

Abstract

Mixed-species bird flocks (hereafter, “flocks”) are an interactive community of largely insectivorous birds, which move and forage together to gain enhanced access to resources and protection from predation. In this study, I aimed to investigate a question: do interspecific interactions within mixed-species flocks change along an elevational gradient? To test this, I carried out this study in the Garhwal region of the Western Himalaya in Dehradun District, Uttarakhand from January to April 2021 (spanned across two seasons). The study areas comprised of three field sites along an elevation gradient from western part of the Rajaji Tiger Reserve (300 m ASL) to Mussoorie (2200 m ASL). I sampled flocks and relative abundances of birds on the two pre-existing trails in each of the three field sites. I walked each trail 10 to 13 times. I recorded species identity, group/cluster size of all birds seen inside as well as outside flocks on the trail and within 10 m on either side of the trail. I assessed arthropod prey availability in winter for the flocking species by using branch bagging technique on the same trails. In total, I clipped 150 branches for the three elevations. I calculated species-specific flocking propensity for each elevation from the information on birds seen within and outside flocks. I used network density (potentially realised associations) and weighted degree (associated strength) to quantify the interspecific interactions (associations) within flocks. To control for the influence of species availability on flock network properties, I generated null flocks using an abundance-based null model. I recorded 80 flocking species in 412 flocks, of which, 179 flocks were observed at the High elevation, 156 at the Mid elevation and 77 at the Low elevation. Majority of species in the Western Himalayan flocks were insectivorous gleaner. I found insect prey availability (resource) to decrease with increasing elevation in winter. As expected, flocking propensity, network density and weighted degree increased with elevation in both seasons indicating greater potentially realised associations and high strength of associations between species. Interestingly, the Low elevation flocks disintegrated completely at the onset of summer possibly due to the greater availability of resources, or an upward migration of species for breeding. I did not analyse the Low elevation flocks in summer due to small sample size (n=5). Flocking propensity and network density decreased from winter to summer at Mid and High elevation while, weighted degrees decreased only for the Mid elevation. This is likely that true summer may not have started at the High elevation during the course of my fieldwork. To conclude, with increasing elevation in both seasons, species not just flocked more, but associated with many species and these associations became stronger, potentially due to the scarcity of resources. It is possible that species may not be able to access resources efficiently when foraging on their own at the High elevation, compelling them to participate in flocks to avail complementary benefits from many other species.