In mixed-species bird flocks (flocks hereafter), participants vary in their degree of similarity with each other. Flock participants can gain group size (supplementary) benefits by choosing similar flock partners, or complementary benefits from dissimilar partners. The nature of benefits, therefore varies based on overall similarity in the flock. Earlier research has shown that flocks world-over tend to be phenotypically clumped and that intraspecifically gregarious species are important benefit providers. In this thesis, I examine changing patterns of associations and species importance with respect to group size, in mixed-species bird flocks.

In my first chapter, I examine the relationship between flock size and phenotypic clumping. I find that small flocks are more phenotypically clumped than expected by chance but as flocks become larger, the phenotypic variation does not differ from what’s expected by chance. I also find that, at a global scale, flocks in regions with lower average flock size are more phenotypically clumped. In the second chapter, I examine the importance of intra-specifically gregarious species. I find that flocks with less than or equal to two gregarious species have lower richness of non-gregarious species than expected by chance. I also study traits of intra-specifically gregarious species that are linked to functional importance and find that individual behavioural traits are not directly correlated to species importance. In the third chapter, I construct emergent networks of flock participant species based on flock co-occurrence. I find that a few species are structurally important in flocks of all sizes, while a few are important only in networks of large flocks. I also find that flock components that are unconnected in smaller flock networks, merge in large flock networks. Overall, I find that species similarity and presence of important species is crucial in smaller flocks whereas large flocks are heterogenous groups that resemble random phenotypic assemblages of flocking birds.