Radar ornithologyStudy of bird movements using the evidence shown on radar screens. Flocks of birds cause traces known as 'angels', which indicate migration across the area covered. Morning movements outwards from a communal roost such as might be occupied by Starlings Sturnus vulgaris form circular patterns called 'ring angels'.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver