A specialised keratinous outgrowth of the skin, which is a unique characteristic of birds. Feathers are highly complex structures that provide insulation, protection against mechanical damage, protective coloration, and also function significantly in behavior. One special functional role is in flight, where feathers provide propulsive surfaces and a body surface aerodynamically suitable for flight. Feathers are used in maintenance of balance and occasionally in the capture of prey and various specialised displays.