Body Down
Down is useful stuff, as it is warm but light. Because the structure of the feather is loose and fluffy, it holds air well.
Adult birds keep some down feathers, lying beneath the contour feathers on the body.
This helps keep bird’s warm, doesn’t weigh them down, and helps keep water bird’s buoyant.
The Source:
Page #6
Ornithology #21
Feathers: The Anatomy
of a Feather #2
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