Birds are in the Animalia kingdom, making them animals. Birds are a member of the phylum Chordata and the subphylum Vertebrata, which means they have vertebrae, and they are in the class Aves.
Birds are defined by their feathers, beaks that have no teeth and their high metabolism. While birds are known for adept flying, penguins, ostriches and many other birds use their forelimbs for purposes other than flight.
The origin of birds has long been a topic of controversy, since they have clear differences with reptiles, mammals and other groups, but research strongly suggests descent from dinosaurs. As a result, some palaeontologists suggest using the term nonavian dinosaurs when discussing the ancient creatures often called dinosaurs.