Is Penguin considered as a bird?

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Penguin originally the common name for the now extinct great auk of the N Atlantic and now used (since the 19th cent.) for the unrelated antarctic diving birds. Penguins, which are related most closely to the albatrosses, are the most highly specialized of all birds for marine life. They swim entirely by means of their flipperlike wings, using their webbed feet as rudders. Their stiff feathers serve as insulation, and are waterproof when oiled. Since their legs are set far back on their bodies, they waddle awkwardly on land, and often travel by tobogganing on their bellies over the ice as they migrate—sometimes great distances—each fall to their nesting sites. Underwater they can swim up to 25 mi (40.3 km) per hr as they pursue the fish, squid, and shrimp that form their diet. They do not eat while on land, subsisting on a layer of fat under the skin; this results in weight losses of up to 75 lb (33.8 kg) during the two-month incubation period. Their chief enemies are the leopard seal, killer whale, and skua gull. Penguins are highly gregarious, and a population density of half a million birds in 500 acres has been counted at a colony in Antarctica. There are 17 species of penguins, 10 of which are considered endangered or threatened. The largest penguins, the emperor and the king (3–4 ft/91.5–122 cm in height), incubate their eggs between their feet in a fold of skin. The smaller jackass penguins, Spheniscus demersus, are named for their braying cry, and crested penguins are distinguished by yellow plumes on either side of the head. Smallest of all is the little blue penguin, Eudyptula minor, of New Zealand and Australia. Penguins are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Sphenisciformes, family Spheniscidae.