Class Aves consists of all of the different birds. Birds are characterized by having feathers, forelimbs without claws that have evolved into wings, and a lack of teeth. All parts of birds are modified to fly. For instance, the feathers of birds have hollow shafts and they have few vertebrae in the tail, both adaptations for less weight. Connecting barbules in the feather provide and strong breast muscles provide rigidity and strength needed for flight. However, each bird has wings specified for a different task. Some birds flap repeatedly to remain in flight, while others are adapted to soaring on wind currents.

Other birds, like the ostrich and emu, don't fly at all, but probably evolved from an ancestor who did but lost the ability because they had no predators. It is believed that birds evolved from two-legged reptiles 150-200 million years ago. Amniotic eggs, scales on the legs, keratin toenails, and even feathers (which evolved from scales) are parts of the bird that evolved from reptiles. A main difference between birds and reptiles is that birds are endothermic, while reptiles are ectothermic. This means that birds heat themselves, while reptiles obtain heat from their external environment. Birds are easily able to heat themselves because of a high rate of metabolism. There are currently 27 orders of birds, with about 9,000 species.