Bird Feathers
Feathers are complex structures found on the outside of the body of birds, and some dinosaurs as well. Not all feathers are used for flight. Feathers insulate birds from water and cold temperatures. They may also be plucked to line the nest and provide insulation to the eggs and young. The individual feathers in the wings and tail play important roles in controlling flight.
Although feathers are light, a bird's plumage weighs two or three times more than its skeleton, since many bones are hollow and contain air sacs. Color patterns serve as camouflage against predators. Striking differences in feather patterns and colors serve to differentiate the sexes.
Some birds have a supply of powder down feathers which grow continuously, with small particles regularly breaking off from the ends of the barbules. These particles produce a powder that goes through the feathers on the bird's body and acts as a waterproofing agent and a feather conditioner. Waterproofing can be lost by exposure to emulsifying agents due to human pollution. Feathers can then become waterlogged, causing the bird to sink. It is also very difficult to clean and rescue birds whose feathers have been fouled by oil spills. 
The colors of feathers are produced by pigments, by microscopic structures that can refract, reflect, or scatter selected wavelengths of light, or by a combination of both. Most feather pigments are melanins (brown and beige pheomelanins, black and grey eumelanins, and carotenoids (red, yellow, orange). Some birds show structural coloration or iridescence, with the blues and bright greens of many parrots being produced by constructive interference of light reflecting from different layers of structures in feathers.
It was thought that feathers evolved as per the needs for insulation, flight and display. Discoveries of non-flying late Cretaceous feathered dinosaurs in China, however, suggest that flight could not have been the original primary function as the feathers simply would not have been capable of providing any form of lift. There have been suggestions that feathers may have had their original function in thermoregulation, waterproofing, or even as sinks for metabolic wastes such as sulphur.
Recent discoveries claim that feathers supported a thermoregulatory function, at least in smaller dinosaurs. The number of feathers per unit area of skin is higher in smaller birds than in larger birds, and this trend points to their important role in thermal insulation, since smaller birds lose more heat due to the relatively larger surface area in proportion to their body weight.
The miniaturization of birds also played a role in the evolution of powered flight. The presence of a certain type of keratin (which is what feathers and scales are made of) in both birds and crocodilians indicates that it was inherited from a common ancestor. This may suggest that crocodilian scales, bird and dinosaur feathers, and pterosaur pycnofibres may have all come from the same primitive skin structures.