A bird is an animal of the class Aves. The primary distinguishing characteristic is the presence of feathers. Other common characteristics are the ability to fly using wings (with a few exceptions), a bony beak with no teeth, the laying of hard-shelled eggs, high metabolism, and a light but strong skeleton. Some birds have a very long beak like the toucan. In general, birds are the most extensively studied animal group, with hundreds of academic journals devoted to their study.
Birds range in size from the tiny hummingbird to the large ostrich and emu. Flightless birds include the penguin, ostrich, and the extinct dodo -- most flightless species have been hunted to extinction, e.g. the Auk, flightless rails, and a giant New Zealand species. Birds are a very differientiated class with some feeding on nectar, seed, insects, rodents, fish, carrion, other birds: Sibley & Monroe's Phylogeny and Classification of Birds (~1990) is a landmark work on the matter. Some birds are nocturnal. Some birds migrate long distances to utilize marginal habitats (e.g. Arctic Tern) while others spend almost all their time at sea (e.g. Wandering Albatross).
Birds are an important food source for humans. This comes mainly from chickens and chicken eggs although goose, pheasant, duck, emu, ostrich, pigeon, grouse, quail, dove, and others are utilized. At one time swan and songbirds were delicacies of the rich and powerful although these are generally protected now. Many species that were once popular on the menu are now extinct, e.g. Passenger Pigeon.
Homing pigeons are used to carry messages and the honeycreeper leads some Africans to honey. Oxpeckers live symbiotically with water buffalos. Numerous species are commensal on man and have become pandemic: Rock Doves (Columba livia), House Sparrows (Passer domesticus), European Starlings (Sturnus vulgaris). Colorful tropical birds (e.g. Parrots, Minahs) are often kept as pets although this has lead to smuggling of some endangered species; CITES does considerable work to deter this.
Some bird species (list to be moved somewhere appropriate when longer): tufted puffin