Commensal Feeding

Some birds eat alone. For those that do not, the choice of a dining partner can be surprising. Why should a phalarope associate with an avocet, or a coot with a swan? Often, it is because a feeding association benefits the participants by enhancing foraging success while increasing protection from predators. For example, by simply standing close to a foraging White Ibis, a Great Egret can snatch stray prey scared to the surface by the ibis but beyond the ibis' reach. In return, the egret warns the shorter, less wary ibis of predators. But not all foraging associations are mutually beneficial.

In commensal associations, members of one species assist the foraging of another, but incur no significant costs and receive no benefits. One of the more common commensal associations involves "beaters," which stir up prey, and "attendants," which simply follow in their footsteps taking whatever comes their way. Many waterbirds, marsh birds, and shorebirds attend particular beater species. Great and Snowy Egrets, for example, attend cormorants; Snowy Egrets, Tricolored Herons and Great Egrets attend mergansers. Some attendants will follow more than one beater species. Enterprising American Coots attend Canvasbacks, Tundra Swans, Mallards, pintails, and Redheads. In water of swimming depth, Wilson's Phalaropes will follow Northern Shovelers; where they can wade, they will often forage behind American Avocets.

Beater-follower associations are not restricted to waterbirds, of course. On land, Cattle, Snowy, and Great Egrets attend cattle, European Robins follow wild pigs, antbirds follow army ants, and African drongos (jay-sized insectivores) follow many species of mammals and birds in anticipation of insects flushed by the "beaters." Interestingly, an African drongo can be sustained following a single elephant, but when following small antelopes, it requires a small herd.

The distance separating attendants from their beaters is not uniform. It depends on the habitat, the type of prey and the ease of its capture, and the speed of the feeders. Consequently, it is not always easy to determine whether two birds seen near each other are feeding commensally. In his study of slow-walking Little Blue Herons following White Ibises, ornithologist James Kushlan compared the foraging success of "attending" (venturing to within one meter of the ibis beater) and "independent" (staying farther away) heron individuals. Kushlan found that attending herons caught twice as many prey as those feeding alone and that the increase reflected more frequent feeding attempts (presumably because the beater stirred up more prey), rather than more successful feeding attempts.

Commensal feeding arrangements can also involve food recycling. In New Guinea, the diet of Shining Starlings includes fruits with large, hard pits. The starlings digest the fleshy coating but regurgitate the pit. Opportunistic Emerald Doves, whose strong stomachs are able to grind tough materials, take in these stripped pits and digest them. Sparrows and finches that feed on seeds in horse manure provide a similar example.

Three of the more common forms of commensal feeding in North American woodlands involve woodpeckers. Some hummingbirds, warblers, and kinglets drink sap oozing from sapsucker "wells" (holes drilled into trees by the sapsuckers), and other species, including bluebirds and nuthatches, follow insect-seeking woodpeckers to snap up prey they miss. After Pileated Woodpeckers clear the outer bark from a section of tree trunk, Hairy Woodpeckers, which are not bark removers, may seek insects then exposed close enough to the surface to exploit. In the case of these woodpeckers, however, it has yet to be demonstrated that the associations exact no cost (in missed food) to the producer.

There may be no out-and-out exceptions to the ecological slogan, "There is no free lunch," but while not obviously damaging to their benefactors, some species definitely gain from the actions of others with whom they forage.

SEE: Mixed-Species Flocking; Coevolution; Bird Communities and Competition; Interspecific Territoriality.

Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.