Higher-level evolution of intraspecific flock-feeding in birds

Intraspecific flock-feeding occurs in many species of birds but the ecological factors that promote the behavior are not clear. Adaptive hypotheses suggest that such flocking should be more likely in species that are vulnerable to predation due to their smaller size, their preference for more open habitats or their more active foraging mode. In addition, flocking is also thought to be promoted by prey clumpiness because clumps are more easily located by foragers in groups. I examined the relationship between ecological traits and the evolution of flocking at the family taxonomic level among all bird families using a phylogenetic approach. Flocking evolved more often in clades with a preference for clumped prey but also occurred with dispersed prey in groups with kin bonds. The role of predation effects was less clear. The likelihood of flocking actually increased with body mass and failed to correlate with habitat openness in the phylogenetically corrected analysis. Although flocking prevailed in clades with an active foraging mode, all clades with a non-active foraging mode showed a preference for dispersed prey, which on its own could account for the effect of foraging mode. The same effect could explain the relative lack of flocking among clades with nocturnal habits. Flocking occurred to the same extent across habitat types and climate zones. At the family level, the results so far indicate a major role for prey characteristics in the evolution of intraspecific flock-feeding in birds. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/ s00265-002-0461-7.