Generally speaking a bird is any member of the class known as Aves that share certain common characteristics and traits. Birds are warm-blooded, bipedal animals whose anatomy is characterized by forelimbs modified through natural selection and evolution to become wings, whose exterior is covered by feathers, and that have, in most cases, hollow bones to assist in flight.
Most birds are diurnal, or active during the day, but some are nocturnal, active during the evening hours, such as owls, and still others feed either day or night as needed.
Many birds migrate long distances to find the optimum or ideal habitats, while others rarely range from their original breeding spots.
Shared characteristics of birds may include a bony or hard beak with no teeth, the laying of hard-shelled eggs, a light but strong skeleton, and a high rate of metabolism.
Most birds are characterized by flight although several well-known species, particularly those that reside on islands, have now lost this ability. Some common flightless birds include the ostrich, penguin, kiwi and now extinct Dodo.
Birds feed on plants, seeds, insects, fish, carrion or other birds. Birds are also an important food source for humans. The most commonly eaten species is the domestic chicken, although geese, pheasants, turkeys and ducks are also common fare, particularly around Thanksgiving Day and the holidays. Birds grown for human consumption are known as poultry. Humans have caused the disappearance of some species due to habitat destruction, hunting or over consumption.
Other species of birds have come to depend on human activities for food and are so widespread as to be considered a nuisance such as the common pigeon or rock pigeon. In North America, sparrows, starlings, and finches are also widespread. Some birds have been used by humans to perform tasks, such as homing pigeons in the days before modern communications, and falcons to aid in hunting or for sport. Tropical birds are often sought after and kept as pets although some are now listed as endangered and their trafficking for this purpose has been restricted.
The bird population, like many other fish and wildlife groups, is facing threats worldwide. According to World watch Institute, bird populations are declining, with 1,200 species facing extinction in the next century. Among the most prevalent reasons cited are habitat loss, predation by nonnative species, oil spills and pesticide use, climate change and excessive rates of hunting and fishing. All these threats make it ever more important to understand, appreciate and protect the birds we see around us everyday.

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