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feathers, outgrowths of the skin, constituting the plumage of birds. Feathers grow only along certain definite tracts (pterylae), which vary in different groups of birds. Feathers develop from tiny projections of tissue (papillae) embedded in follicles and nourished by blood vessels in the dermis. When the feather is full grown, the blood supply is discontinued and the central shaft becomes hollow. A secretion of the thyroid gland stimulates the papilla to develop a new feather when one has been molted or pulled.
In a typical feather, barbs extend outward from the distal portion of the shaft, or rachis; smaller crosslinking barbules and hooks interlock neighboring barbs, forming a web that gives the feather both strength and flexibility. Down feathers, or plumulae, the first plumage of young birds and the protective undercoat of aquatic birds, lack these interlocking projections. Specialized feather forms are found in crests, top-knots, ruffs, and tail feathers. Bristles are modified feathers. The colors red, yellow, brown, and black are caused by pigment in the feathers. There are no blue pigments, and green and violet are rare; however, these colors, as well as iridescent effects, are caused by the reflection and diffraction of light.
Feathers are lightweight, durable, and in some cases waterproof. They have protective and decorative functions, but, aside from their role in bird flight, their most important capacity is heat retention. They are believed to have evolved from reptilian scales in Mesozoic times, but little is definitely known about how they arose. Feathers have been used by humans from ancient times for millinery and other ornamental purposes. The indiscriminate hunting of certain birds for their feathers has resulted in their near extinction; it is now prohibited by law in the United States.
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