Feathers are outgrowths of the skin which are nourished by blood vessels. Once the feather is fully grown, the blood supply is cut off and the central shaft becomes hollow. A complex structure, the typical feather includes a shaft, vanes, barbs, and barbules. Sometimes there is an aftershaft as in the spruce grouse. Down is a specialized feather which provides effective insulation.
All feathers also provide heat retention and some are waterproof.
Feather colour, shape and wear can be used to determine age or sex. During the breeding season, male plumage may be brightly coloured to help attract females. In the winter some northern birds such as gray jays grow additional feathers to keep warm. Preening involves the careful cleaning, rearrangement and oiling of the feathers with the bill. It is essential for the bird's survival.