Birds include any warm-blooded vertebrate of the class Aves, having a body covered with feathers, forelimbs modified into wings, scaly legs, a beak, and no teeth, and bearing young in a hard-shelled egg.

Birds have a beak, which is a hard bill covering the jaw, and a four-chambered heart.

It is generally believed that birds are descended from dinosaurs and probably evolved from them during the Jurassic Period. While most paleontologists believe that birds evolved from a small dinosaur called the theropod, which in turn evolved from the thecodont, a reptile from the Triassic Period, other paleontologists believe that birds and dinosaurs both evolved from the thecodont. There are some who even consider the bird to be an actual dinosaur. According to this view, the bird is an avian dinosaur, and the older dinosaur a nonavian dinosaur. Although there are variations of thought on the exact evolution of birds, the similarities between birds and dinosaurs are striking and undeniable. Small meat-eating dinosaurs and primitive birds share about twenty characteristics that neither group shares with any other kind of animal; these include tubular bones, the position of the pelvis, the shape of the shoulder blades, a wishbone-shaped collarbone, and the structure of the eggs. Dinosaurs had scales, and birds have modified scales—their feathers—and scaly feet. Some dinosaurs also may have had feathers; a recently discovered fossil of a small dinosaur indicates that it had a featherlike covering. In fact, some primitive fossil birds and small meat-eating dinosaurs are so similar that it is difficult to tell them apart based on their skeletons alone.