Birds, a subclass of reptiles, are the last dinosaurs to survive. They are a group of endothermic vertebrates characterized by feathers, toothless beaks and jaws, egg laying hard shell eggs, high metabolic rate, four chamber heart and strong and light skeleton. Birds live worldwide, ranging in size from 5 cm (2 in.) hummingbirds to 2.75 m (9 ft.) ostriches. They are the most species of quadruped, about 10000 species, more than half of which are Passeriformes, sometimes called roosting birds. Birds are close relatives of crocodiles.

Fossil records show that birds evolved from feathered ancestors of the theropods of sauropods. Real birds first appeared in the Cretaceous period about 120 million years ago. Based on DNA evidence, it was found that at the time of the Cretaceous Paleogene extinction 66 million years ago, birds diversified dramatically, reducing pterosaurs and killing all non avian dinosaurs. Birds, especially those from the southern continent, survived the event and then moved to the rest of the world during the global cooling, while diversifying. In the broader avialae group, primitive bird dinosaurs found outside the intrinsic category of Aves can be traced back to the Middle Jurassic about 170 million years ago. Many of these early "stem birds," such as Archaeopteryx, were not yet fully powered, and retained many of the original features, such as a toothed lower jaw instead of a beak and a long bone tail.

The wings of birds are more or less determined by their species. The only known groups without wings are the extinct MOA and elephant birds. The wings evolved from the forelimbs gave the birds the ability to fly, although further evolution has led to the loss of the ability of flightless birds, including flat breasted dogs, penguins and birds of various endemic island species. The digestive and respiratory systems of birds are also particularly suitable for flight. Some aquatic bird species, especially seabirds and some water birds, have evolved to swim.