GapeThe mouth, namely the space between the upper mandible and the lower. The angle of the gape is called the 'commensural point'. An aerial feeder has a particularly wide gape, as does a nidicolous nestling, in the latter case outlined by colourful 'gape flanges', presumably to guide the parent when thrusting in food.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver