Brood patchA bare area on a bird's underside which develops at the time of year when the eggs require incubation. It is completely free of down and other feathers and is well supplied with blood vessels. Birds may have from one to three brood patches according to their species, although some have none, such as the Gannet Sula bassana, which warms its single egg with its webbed feet.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver