DimorphismThe existence of two distinctive forms (usually differing in terms of plumage colour) within a species, but not regarded as constituting separate subspecies. Sexual dimorphism is common, while some species have two 'colour phases' or 'morphs', for example the Fulmar Fulmarus glacialis and the Arctic Skua Stercorarius parasiticus, each of which has dark and light forms. Where more than two phases exist the situation is described as polymorphism.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver