Powder downA powdery substance into which certain tiny body feathers break down, found in members of the heron family (Ardeidae). Presumably it soaks up fish slime from the plumage. Other types of birds also produce powder from their feathers, for example pigeons (family Columbidae), 'powder impressions' of which may be left on windows into which they have accidentally flown.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver