Component structure of the outer covering and flight surfaces of all modern birds. Unique to birds, feathers apparently evolved from the scales of birds' reptilian ancestors. Like hair, feathers are made of keratin, a fibrous protein. They are variously specialized for insulation, flight, formation of body contours, display, and sensory reception. Contour feathers form most of the surface of the bird, streamlining it for flight and often waterproofing it. The basal portion may be downy and thus act as insulation. The major contour feathers of the wing (remiges) and tail (rectrices) function in flight. Contour feathers grow in tracts (pterylae) separated by bare areas (apteria) and develop from follicles in the skin. Down feathers have loose-webbed barbs, all rising from the tip of a very short shaft. Their function is insulation, and they may be found in both pterylae and apteria in adult birds. They also constitute the first feather coat of most young birds. Filoplumes are hairlike feathers with a few soft barbs near the tip. They are associated with contour feathers and may be sensory or decorative in function. Bristlelike, vaneless feathers occur around the mouth, eyes, and nostrils of birds. Some bristles function as eyelashes on ground-dwelling birds; bristles over the nostrils may serve as filters.
Structures of a typical contour feather. The feather's central shaft (rachis) has a series of
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