Not everybody accepts the theory of evolution, but those who do have a theory to explain how birds came to be. There are many different parts of birds that are quite specialized because they have developed the ability to fly. They became warm-blooded because warm-blooded animals can change food into energy quickly. They have feathers, not only to keep warm, but because feathers make lightweight flying surfaces. They lost their teeth, because teeth are made of heavy bone. They have developed legs that act as shock absorbers. Their eyesight is very sharp.