ClumpingHuddling together in a roost in order to conserve body heat. It is characteristic of very small species in winter or when young, examples being the Wren Troglodytes troglodytes and the Long-tailed Tit Aegithalos caudatus. These tiny birds lose heat more quickly than larger ones.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver