Feathers are one of the most prominent features of a bird's anatomy, and they are unique to birds. Every bird has feathers and everything that has feathers is a bird . Feathers perform a number of functions for a bird. It is believed by most scientists that this insulating effect was the primary force driving the evolution of feathers, i.e. ancestral birds developed feathers to keep themselves warm.
Feathers also protect birds from UV light.
Secondly, feathers allow for flight. Scientists believe that flight evolved in birds as a result of their possessing basic feathers and that this added selective pressure to the evolution of feathers making them larger, stronger and refining their structure. Thirdly, feathers control what a bird looks like. A plucked chicken or pigeon looks very different to a fully feathered one. Feathers supply the bird with colours allowing for camouflage and secondary sexual characteristics and sexual display.