Mixed flocks and polyspecific associations: Costs and benefits of mixed groups to birds and monkeys.

This review examines the diversity of avian mixed foraging flocks with the goal of relating the conclusions to primate polyspecific associations. Mixed associations are considered as adaptations for achieving an optimal balance between predator protection and feeding efficiency. In open habitat, predator and prey are able to detect each other at a distance and feeding competition is low, especially in species that subsist on a homogeneously distributed food supply. These conditions favor large groups of variable composition. In closed habitats, predators attack at close range, so early warning alarm systems are at a premium. Feeding competition is often intense because food resources such as fruit, flushing leaves, and nectar are spatially concentrated. Since feeding competition is generally less between than within species, these conditions favor mixed associations composed of small numbers of several to many species, and the evolution of elaborate early warning systems to thwart predators. The primate polyspecific associations that have been studied to date share characteristics with the closed habitat model while exhibiting some important distinctions. Primate associations are made up of integral troops, not individuals, implying high incremental costs of joining. These costs, plus a paucity of ecologically compatible combinations of species, seem to limit primate polyspecific associations geographically to regions in which the presence of monkey-eating raptors provides a strong incentive for aggregation.