Any of the warm-blooded, beaked vertebrates of the class Aves, including more than 9,600 living species. A covering of feathers distinguishes birds from all other animals. Birds have a four-chambered heart (like mammals), forelimbs modified into wings, and keen vision, and their eggs have calcium-rich eggshells. Their sense of smell is not highly developed. Birds are found almost worldwide in diverse habitats on both land and water. Dietary preferences and nest structure vary widely. Almost all species incubate their eggs. Flying birds have evolved skeletons in which part of the bone is replaced by air spaces, an adaptation for reducing weight. The crop, an enlarged part of the esophagus used for temporary food storage, enables birds to feed while in flight. Humans use wild and domesticated birds and their eggs for food, hunt wild birds for sport, and use feathers for decoration and insulation. More than 1,000 extinct species of bird have been identified from fossil remains; the earliest fossil bird known is Archaeopteryx.