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. Feathers are variously specialized for insulation, flight, formation of body contours, display, and sensory reception. Unlike the hair of most mammals, feathers do not cover the entire skin surface of birds but are arranged in symmetrical tracts alternating with areas of bare skin, which may contain the small, soft feathers called down. A typical feather consists of a central shaft (rachis), with serial paired branches (barbs) forming a flattened, usually curved surface
the vane. The barbs possess further branches, the barbules, and the barbules of adjacent barbs are attached to one another by hooks, stiffening the vane.