UpendingImmersing the forepart of the body, head downwards, in the water, thus causing the hind part to stick up into the air. This position is maintained by paddling movements of the feet. Among aquatic birds upending is likely to be practised by a surface feeder, in order to increase the depth to which it can reach.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver