Perches

Birds are bipedal, warm-blooded, egg-laying vertebrates characterized primarily by feathers, forelimbs modified as wings, and hollow bones. more...

Birds range in size from the tiny hummingbirds to the huge Ostrich and Emu. Depending on taxonomic viewpoint, there are about 8,800–10,200 living bird species (plus about 120–130 that have become extinct in the span of human history) in the world, making them the most diverse class of terrestrial vertebrates.

Birds are a very differentiated class, with some feeding on nectar, plants, seeds, insects, rodents, fish, carrion, or other birds. Most birds are diurnal, or active during the day. Some birds, such as the owls and nightjars, are nocturnal or crepuscular (active during twilight hours). Many birds migrate long distances to utilise optimum habitats (e.g., Arctic Tern) while others spend almost all their time at sea (e.g. the Wandering Albatross). Some, such as frigatebirds, stay aloft for days at a time, even sleeping on the wing.

Common characteristics of birds include a bony beak with no teeth, the laying of hard-shelled eggs, high metabolic rate, and a light but strong skeleton. Most birds are characterised by flight, though the ratites are flightless, and several other species, particularly on islands, have also lost this ability. Flightless birds include the penguins, ostrich, kiwi, and the extinct Dodo. Flightless species are vulnerable to extinction when humans or the mammals they introduce arrive in their habitat. The Great Auk, flightless rails, and the moa of New Zealand, for example, all became extinct due to human influence.

Birds are among the most extensively studied of all animal groups. Hundreds of academic journals and thousands of scientists are devoted to bird research, while amateur enthusiasts (called birdwatchers or, more commonly, birders) probably number in the millions.

High-level taxonomy

Birds form a class, whose scientific name is Aves. The founding species of class Aves probably lived in the Jurassic period.

According to the most recent consensus, Class Aves and a sister group, the family Crocodylidae, together form a group of unnamed rank, the Archosauria.

The class of birds separated early into two superorders, the Paleognathae (mostly flightless birds like ostriches), and the wildly diverse Neognathae, containing all other birds.

Bird orders

This is a list of the taxonomic orders in the class Aves. The list of birds gives a more detailed summary, including families.

• Struthioniformes, Ostrich, emus, kiwis, and allies
• Tinamiformes, tinamous
• Anseriformes, waterfowl
• Galliformes, fowl
• Sphenisciformes, penguins
• Gaviiformes, loons
• Podicipediformes, grebes
• Procellariiformes, albatrosses, petrels, and allies
• Pelecaniformes, pelicans and allies
• Ciconiiformes, storks and allies
• Phoenicopteriformes, flamingos
• Accipitriformes, eagles, hawks and allies
• Falconiformes, falcons
• Turniciformes, button-quail
• Gruiformes, cranes and allies
• Charadriiformes, plovers and allies
• Pteroclidiformes, sandgrouse
• Columbiformes, doves and pigeons
• Psittaciformes, parrots and allies
• Cuculiformes, cuckoos
• Strigiformes, owls
• Caprimulgiformes, nightjars and allies
• Apodiformes, swifts
• Trochiliformes, hummingbirds
• Coraciiformes, kingfishers
• Piciformes, woodpeckers and allies
• Trogoniformes, trogons
• Coliiformes, mousebirds
• Passeriformes, passerines

Note: This is the traditional classification (the so-called Clements order). A more recent, radically different classification based on molecular data has been developed (the so-called Sibley order) and is gaining acceptance.

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