TarsusThe part of a bird's leg between what appears to be a backward-facing 'knee' and what appears to be an 'ankle'. In fact the bird's true knee is always hidden under the feathers, and the apparent 'knee' (the 'tarsal joint') is the ankle and heel. Therefore the tarsus is really the part of the foot between the heel and the ball, so that a bird stands on its toes. The tarsus may also be called the 'shank', and its bone is called the 'tarsometatarsus'.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver