Feathers are believed to have evolved from reptilian scales in Mesozoic times. When the feather is full grown, the blood supply is discontinued and the central shaft becomes hollow. Down feathers, or plumulae, the first plumage of young birds and the protective undercoat of aquatic birds, lack these interlocking projections. A secretion of the thyroid gland stimulates the papilla to develop a new feather when one has been molted or pulled. 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. 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. Feathers have been used from ancient times for millinery and other ornamental purposes. The Columbia Encyclopedia, Sixth Edition.