They still share some characteristics with reptiles, such as laying eggs and having scales on their legs and feet. Development of the ability to fly required not only feathers and wings but good eyesight, a sense of balance and fine muscle coordination. Birds are warm-blooded vertebrates. Like mammals, birds are warm-blooded, meaning their internal body temperature is maintained at a constant level regardless of external conditions. This characteristic allows birds to maintain high levels of energy and a metabolic rate necessary for flight.
Feathers are an adaptation of reptilian scales.
The feathers most commonly observed are contour and down feathers. The contour feathers on the tail and wings have been modified for flight. Down feathers are small and lie under the contour feathers. The purpose of these feathers is to insulate the bird from the cold and protect against sunburn. Birds must take care of their feathers so they can continue to fly and remain warm.