ImmatureA young bird in the first plumage after moult of its juvenile feathers. In many birds the immature plumage is very similar to that of the adult but in some, such as the Gannet Sula bassana, the immature passes through a whole series of plumages before reaching adulthood.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver