Birds are vertebrates of the class Aves and are unique in having feathers, the one major characteristic that distinguishes them from all other animals. Birds also have beaks and forelimbs modified into wings. Most birds are able to fly, but some, like ostriches and penguins, cannot (even though they still have wings).

Like mammals, birds are warm-blooded and have a four-chambered heart. All birds reproduce by laying eggs with hard shells, and most birds build nests to protect the eggs from weather and predators. Adult birds almost always sit on the eggs to keep them warm until they hatch. Birds can be found in all habitats above ground, and there are even some species that make nests in underground burrows. Visit the Philadelphia Zoo's colorful birds in Bird Valley and throughout the Zoo in exhibits like the McNeil Avian Center.

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