Mixed-species feeding flock

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A mixed-species feeding flock or mixed hunting party is a flock of birds of different species that join each other to search for food.

A proverb says, "Birds of a feather flock together," but birds of different kinds often occur together. They may do so at rich food sources (as animal carcasses, termite hatches, garbage dumps, fruiting or flowering trees, schools of fish) or simply because they share habitat and tolerate each other, as many shorebirds, gulls, ducks, starlings, and icterids do.

They may also travel together. Many seed-eating birds (finches, sparrows, buntings, etc.) feed in mixed flocks, often with closely related species, in grassland or scrub. However, mixed feeding flocks are particularly common in forests. In the North Temperate Zone, they are typically led by tits and chickadees, often joined by nuthatches, treecreepers, woodpeckers such as the Downy Woodpecker and Lesser Spotted Woodpecker, kinglets, and (in North America) New World warblers—all insect-eating birds. They are particularly common outside the breeding season.

Insectivorous feeding flocks reach their fullest development in tropical forests, where they are a typical feature of bird life. In the Neotropics the leaders or "core" members may be Black-throated Shrike-Tanagers in southern Mexico, Three-striped Warblers in Central America, and antbirds such as the Bluish-slate Antshrike in South America. Core species often have striking plumage and calls that attract other birds. Other members may come from most of the region's families of small diurnal insectivorous birds; exceptions include swifts, swallows, and gnateaters.

However, not all birds of these families join mixed flocks. There are genera such as vireos in which some species join mixed flocks and others don't. Of the three subspecies of Yellow-rumped Warbler, only one (Audubon's) typically does. Some species appear to prefer certain others: jays of the genus Cyanolyca flock with Unicolored Jays and Emerald Toucanets. Many icterids associate only with related species, but the western subspecies of the Yellow-backed Oriole associates with jays and the Band-backed Wren.

Flocks wander at about 0.3 kilometers per hour through the forest, with different species in their preferred niches (on the ground, on trunks, in high or low foliage, etc.) Some species follow the flock all day, while others (such as the Long-billed Gnatwren) join it only in their own territories.

Mixed-species flocks on other continents resemble those of the Neotropics. The core members in Africa are often tits; in Asia babblers, drongos and Green Ioras; in Australasia members of the family Pardalotidae such as gerygones in New Guinea and fairy-wrens and thornbills in Australia. As in the Americas, they are joined by birds of other families such as minivets and bulbuls.

In tropical Asia, where this phenomenon is arguably best developed, flocks may number several hundred birds spending the entire day together, and an observer in the rain forest may see virtually no birds except when encountering a flock. For example, as a flock approaches in the Sinharaja Forest Reserve in Sri Lanka, the typical daytime quiet of the jungle is broken by the noisy calls of Orange-billed Babblers and a Greater Racket-tailed Drongo. As the birds pass, the observer can glimpse the quieter, more inconspicuous, members of the flock. If the flock crosses a track, its true numbers become clearer. The two Laughing Thrushes become ten, and the previously missed small species, like Kashmir Flycatcher and Velvet-fronted Nuthatch, reveal themselves.

Though the Sinharaja reserve is large and ecologically important, most of the passerines there are concentrated into six or seven mobile flocks. In smaller patches of jungle, nearly all may occur in just one flock.

The advantages of this behavior are not certain, but evidence suggests that it confers some safety from predators, especially for the less watchful birds such as vireos and woodpeckers, and also improves feeding efficiency, perhaps because arthropod prey that flee one bird may be caught by another (Ehrlich et al.).

[edit] References

Howell, Steve N. G., and Sophie Webb (1994). A Guide to the Birds of Mexico and Northern Central America. Oxford University Press. ISBN 0-19-854012-4.