Body mass, diet, foraging strata and foraging behaviour predict variation in the propensity to join mixed-species flocks across birds

Mixed-species flocking is common in birds and is associated with increased foraging efficiency and reduced predation pressure. The propensity to join mixed-species flocks within a community varies across species, possibly reflecting species-specific needs for increased protection from predators, opportunities to increase foraging efficiency or both. Previous studies on predictors of mixed-species flocking propensity across species have typically focused on local communities, with or without accounting for phylogenetic relatedness. Here, we examined predictors of mixed-species flocking propensity across the published literature using a phylogenetic linear mixed model framework. We obtained 724 mixed-species flocking propensity estimates from 31 different study sites located primarily in South America and East Asia. Flocking propensity was associated with diet, foraging strata and foraging behaviour, and was inversely related to body mass. Controlling for phylogeny and body mass, flocking propensity was significantly lower for nectarivores than for species with other diets, lower for species foraging on the ground than in higher strata, and lower for sallying species than for bark and near-perch foragers. These results suggest that variation across species in the tendency to join mixed-species flocks can be predicted from knowledge about various species-specific ecological traits in birds.