In ornithology, the patch of feathers covering the vent or cloaca of a bird, located at the base of the tail.
From Latin crissare (to move the haunches). The term became specialized in ornithological vocabulary during the 18th-19th centuries as scientists developed precise anatomical terminology for bird anatomy.
Birdwatchers use the crissum as an identifying feature because its color and markings vary distinctly between species—it's like a hidden field mark that separates the experts from casual observers.
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