Birds are in their own grouping and are not reptiles or mammals because they lack mammary glands and are not cold blooded. The amphibians, fish, invertebrates, mammals, and reptiles are the other five other basic animal groups, and the bird group is one of them.
Birds possess a variety of distinctive traits in addition to feathers. Birds have a four chambered heart and a rapid metabolic rate. They have light-weight bones and lay eggs. Approximately 150 million years ago, during the Mesozoic Era, scientists estimate that birds first appeared. Today, there are more than 9,000 different bird species worldwide.