From flocking propensity to within-flock association: A two-stage framework for mobile interaction networks in mixed-species bird flocks

Many ecological networks are dynamic, yet the traits associated with participation may differ from those structuring associations among participants. Mixed-species bird flocks (MSFs) provide a tractable system for testing this possibility. We evaluated a two-stage framework in which traits shape (i) species’ flocking propensity and (ii) within-flock association strength. We combined multi-year surveys across five landscapes in China spanning primary and secondary forest, with museum-based plumage reflectance data quantified under avian visual models—a biologically explicit approach rarely applied in MSF studies—and phylogenetic data. At the first stage, flocking propensity was strongly phylogenetically structured. Trunk-foraging and smaller-bodied species had higher flocking propensity, whereas hand-wing index (HWI), a proxy for maneuverability, showed a weaker, uncertain effect. Whole-body plumage dichromatism showed no clear effect, although body-part-specific analyses revealed a localized positive effect of forewing achromatic dichromatism. At the second stage, within-flock association strength was greater among species that were more similar in body size and HWI, shared diet and foraging stratum, and were more closely related. HWI similarity was the most consistently supported predictor, with support in eight of ten local networks. Whole-body plumage similarity showed no consistent effect, although localized body-part-specific signals emerged. Overall, the two stages showed partly distinct trait patterns: flocking propensity was associated primarily with body size and foraging stratum, whereas within-flock association strength was structured mainly by functional and phylogenetic matching. This two-stage framework, coupled with avian visual modeling, provides a general approach for understanding how mobile interaction networks assemble across taxa.