Students will be able to identify the four characteristics by which birds are defined. There are more than 9,000 species of birds in the world, with about 800 found in North America. Birds evolved from small reptiles more than 160 million years ago. 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.