Birds (Aves) are a group of animals with backbones. They evolved from dinosaurs. Technically, birds are dinosaurs.[2] The word for \"bird-like\" is avian. Birds are warm blooded. Their feathers help prevent loss of body heat.[3][4] Modern birds do not have teeth. They have beaked jaws. Birds lay hard-shelled eggs. They have a high metabolic rate and a strong but lightweight skeleton. Their hearts have four chambers. Birds live all over the world. They range in size from the 5 cm (2 in) bee hummingbird to the 2.70 m (9 ft) ostrich. There are about ten thousand species of birds. More than half of these are passerines, or perching birds.