Cooperative Roles in Mixed Flocks of Antwrens (Formicariidae)

Abstract

INTERSPECIFIC flocks of birds, so characteristic of the tropics although also occurring in temperate zones, often show features related to exploiting particular food resources, to reducing the risks of predation, and to maintaining flock organization by means of communication. Reviews by Rand (1954), Short (1961), and Moynihan (1962) suggest that flocking has potentially counteracting effects both on the efficiency of foraging and on the risks of predation. Under certain conditions, flocking might enhance the foraging efficiencies of some or all the participating individuals. For instance, individuals might capture insects flushed by other members of the flock (Swynnerton, 1915; Brosset, 1969), or flocks might exploit patches of abundant food more efficiently than solitary individuals could. Yet flocking also increases the chances of competition for food among flock members. Presumably in order to reduce this competition, related species in flocks often clearly differ in their feeding habits, especially in flocks of insectivorous birds (Moreau, 1948; Willis, 1966a, 19,66bi; Vuilleumier, 1967; Morse, 1967, 1969; Brosset, 1969). Flocks composed of several species with different feeding habits might well generate less competition among the participating individuals than would single-species flocks of the same size (Moynihan, 1962). Flocking might also increase the efficiency of detecting, mobbing, or distracting predators (Miller, 1922; Tinbergen, 1951: 168-170; Kruuk, 1964). Yet a flock probably draws the attention of predators more than solitary individuals would, especially if the flock includes noisy or visually conspicuous species (Stresemann, 1917). Nevertheless, visual and auditory signals are critical for maintaining contact among the members of a flock, for attracting recruits to a flock, or for spacing individuals (Moynihan, 1960, 1962). Conspicuous colors, flash patterns, sharp calls, and restless movements, all of which facilitate locating and recognizing the individual bird, thus have a double effect. In addition to promoting flock integration, they must also attract predators. In general , adaptations that reduce the risks of predation, as well as those that reduce competition for food among flock members, should facilitate the evolution of flocking behavior. Yet flocks must compromise between adaptations that reduce predation and those that increase effective communication among flock members.