CapA patch of colour covering the crown. A cap may extend below the eye, in which case it may almost become a hood, and it may also extend on to the nape or hind neck. Examples of caps can be found in most terns (family Sternidae), which have black ones, and in the red cap of the Green Woodpecker Picus viridis.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver