COOPERATIVE ROLES IN MIXED FLOCKS OF ANTWRENS (FORMICARIIDae)

Under certain conditions, flocking might enhance the foraging efficiencies of some or all the participating in- dividuals. 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 feed- ing habits, especially in flocks of insectivorous birds (Moreau, 1948; Willis, 1966a, 1966b.; Vuilleumier, 1967; Morse, 1967, 1969; Brosset, 1969). Flocks composed of several species with different feeding habits might well generate less competition among the participating indi- viduals 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 mem- bers 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 recog- nizing the individual bird, thus have a double effect. In addition to promoting flock integration, they must also attract predators. In gen- eral, 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.

R. H^VEN WILEY, The Auk: Vol. 88, No. 4