PolymorphismThe existence of more than two distinctive forms (usually in terms of plumage) within a species. Each form, which is not regarded as a subspecies, is called a 'morph'. The males of the Ruff Philomachus pugnax are polymorphic, having a great variety of colour and pattern. Species with just two morphs are said to show dimorphism.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver