A singular bare or sparsely feathered area on a bird's skin, particularly between the main feather tracts (pterylae).
From Greek a- (without) + pterion (feather), with Latin singular -ium suffix. This ornithological term describes individual featherless patches on bird skin, the singular of apteria.
Every apterium on a bird is like a deliberate design choice—evolution said 'we need this spot bare for flexibility and warmth management'!
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