AxillaThe 'arm-pit' of a bird, where the underwing joins the body. The feathers of this region are called the 'axillaries', and in a few species their colour can be an aid to identification, for example, black in the case of the Grey Plover Pluvialis squatarola.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver