Field characterA feature which is sufficiently distinctive to enable a particular species to be identified by means of it in the field. It may be a plumage characteristic (also called a 'field mark') (such as the red breast of the Robin Erithacus rubecula), a sound (such as the song of the Cuckoo Cuculus canorus), or a habit (such as the tail-cocking of the Wren Troglodytes troglodytes).
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver