A Pomarine Jaeger harries a Herring Gull. ink drawing by Shahid Naeem
Evolution pays no attention to the commandment "Thou shalt not steal." Birds steal from each other just about anything that is not nailed down. They steal mates, nesting material, eggs, and prey. The term "piracy," however, is generally restricted to the harassment of one bird by another in order to force the first to give up food. In scientific jargon, such piracy is referred to as "kleptoparasitism."

Jaegers and skuas (both close relatives of gulls) are classic avian pirates that attack other birds in midair and make them relinquish their food. Gulls themselves also pirate food from other birds, including auks, shorebirds, and other gulls. The victims
of these pirates may have food snatched from their bills, may be forced to regurgitate, or if pursued by jaegers, may occasionally become prey themselves. Jaegers and skuas obtain much of their food in this way, as do frigatebirds. The latter are lightly built to permit acrobatic flight, and possess feathers that are not very resistant to wetting. They are thus dependent on other birds, including gulls, to do their fishing for them.

Piracy is also commonly practiced by some raptors. In one unusual case, a European Sparrowhawk had its food snatched by a Merlin, which, in turn was robbed by a Honey Buzzard (a Eurasian accipiter), which lost it to a Peregrine Falcon. Turkey Vultures are known to force nestling Great Blue Herons to regurgitate their last meal, which is scooped up and later fed to the vultures' own chicks. Raptors get the tables turned on them occasionally, sometimes being forced to give up their catch to birds such as crows and magpies. Apparently this behavior on the part of the passerine pirates is derived from mobbing.

Interestingly, dabbling ducks, including American Wigeons and Gadwalls, often pirate aquatic vegetation from coots. The latter can dive deeper, and thus the ducks can dine on pond weeds that ordinarily would be out of their reach. They also may save energy in some circumstances by pirating rather than dabbling, since American Coots have been observed to simply drop their food plants immediately upon surfacing when approached by a pirate. Although coots can be quite aggressive, they seldom attack the pirates, perhaps because the extensive preening required after agonistic interactions would consume too much time that could be more profitably spent feeding. Coots can be pirates as well as victims; groups of two to five juvenile coots have been known to snatch aquatic vegetation from the bills of diving ducks and swans.

But most groups of birds do not practice piracy. It is unknown in most songbirds and not recorded in pigeons, doves, or game birds. Apparently piracy is a behavior that evolves under rather special ecological circumstances. Most birds seem to have the greatest reproductive success collecting their own food, rather than running the risks of stealing it from others.
SEE: Natural Selection; Dabblers vs. Divers.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.