In the temperate climate zone, mixed-species flocks of birds are mostly a winter phenomenon, occurring after the breeding season, when territorial boundaries are no longer observed. In some wooded areas of North America, for example, chickadees, titmice, nuthatches, kinglets, brown creepers, and woodpeckers travel together in loose flocks, and while the mix is different in the tropics, the underlying principles are the same. Ecologists who have studied mixed flocks believe they offer a number of advantages over single-species aggregations—better detection of predators, for instance, combined with minimal competition owing to species-specific foraging techniques.
Every species in a mixed flock has a preferred food and a specialized hunting method. The acrobatic chickadees hang upside down from branches, peering into cracks and crevices for hibernating arthropods (small invertebrates, including insects) or egg masses, while the tiny kinglets flit and hover at the very tips of the twigs, where heavier birds cannot reach. Downy woodpeckers work their way up the trunks and nuthatches go down, headfirst, each finding food that the other missed. The creepers, like the woodpeckers, ascend the tree in a spiral, but they pay particular attention to the spaces between bark flakes, while the woodpeckers, with their sturdier bills and barbed, spearlike tongues, focus on bugs hidden deeper inside the tree trunk. Species in mixed flocks often seek the same foods, but the ways in which they look, and where they look, are different.
Mixed-species flocks reach their greatest expression in the tropics, especially in the Amazon Basin, where as many as six dozen species have been seen in a single gathering. Each flock is made up of subsets of birds, each with its own role to play. At the core is one or more “nucleus species,” as they are known, usually a mated pair or a family group; by their behavior and calls, they hold the flock together, set the pace for the group and providing cohesion. They are joined by what ecologists call attendant species, some staying with the flock almost full-time, others following for a few hours but then dropping out, and still others joining only briefly while the flock is traversing their territory.