Birds are neither mammals (they don't have mammary glands) nor reptiles (they are not cold blooded), but instead belong in a grouping all their own. The bird group is one of six basic animal groups, with the others being amphibians, fish, invertebrates, mammals and reptiles.
A number of distinct characteristics can be found in birds, beyond having feathers. Birds have a high metabolic rate and a four-chambered heart. They lay eggs and have lightweight bones. Scientists believe that birds evolved approximately 150 million years ago during the Mesozoic Era, and there are now more than 9,000 bird species around the globe.