Birds are the only animals that have feathers.
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.