Birds are bipedal, warm-blooded, egg-laying vertebrates with about 10,000 living species. The fossil record indicates birds evolved from theropod dinosaurs 200 to 150 million years ago, and the earliest known bird is the late Jurassic Archaeopteryx. There is significant evidence that birds evolved from theropod dinosaurs; as more non-avian theropods that are closely related to birds are discovered, the formerly clear distinction between non-birds and birds becomes blurred. A genetically consistent evolutionary tree would classify birds as reptiles, though some scientists would elevate birds, turtles and crocodilians to their own class, on an equal footing with reptiles.

Globally, 1,313 species of birds, or about 13 percent of the total of 10,064 extant described bird species, were deemed endangered or threatened with extinction by the International Union for Conservation of Nature’s 2012 Red List. In the United States, 79 bird species are at risk of extinction.