People have always been fascinated by birds. Birds' marvelous flying ability makes them seem the freest of all animals. Many birds have gorgeous colors or sing sweet songs. The charms of birds have inspired poets, painters, and composers. Certain birds also serve as symbols. People have long regarded the owl as a symbol of wisdom and the dove as a symbol of peace. The eagle has long represented political and military might.
There are about 9,700 species (kinds) of birds. The smallest bird is the bee hummingbird, which grows only about 2 inches (5 centimeters) long. The largest living bird is the ostrich, which may grow up to 8 feet (2.4 meters) tall. The largest bird that ever lived was the elephant bird, which died out hundreds of years ago. It weighed about 1,000 pounds (450 kilograms). Birds inhabit all parts of the world, from the polar regions to the tropics. They are found in forests, deserts, and cities; on grasslands, farmlands, mountaintops, and islands; and even in caves. Some birds, including albatrosses and certain ducks, always live near water. Most such birds can swim. Other birds, especially those in the tropics, stay in the same general area throughout life. Even in the Arctic and the Antarctic, some hardy birds stay the year around. But many birds of cool or cold regions migrate each year to warm areas to avoid winter, when food is hard to find. In spring, they fly home again to nest.
All birds hatch from eggs. Among most kinds of birds, the female lays her eggs in a nest built by herself or her mate or by both of them. The majority of birds have one mate at a time, with whom they raise one or two sets of babies a year. Some birds keep the same mate for life. Others choose a new mate every year. Most baby birds remain in the nest for several weeks or months after hatching. Their parents feed and protect them until they can care for themselves. Other kinds of baby birds, including chickens and ducks, become active and able to walk and feed themselves soon after hatching. Most birds leave their parents after only a few months.
Birds belong to the large group of animals called vertebrates. Vertebrates are animals with a backbone. The group also includes fish, reptiles, and mammals. Birds have two forelimbs and two hindlimbs, as do cats, frogs, lizards, and many other vertebrates. But in birds, the forelimbs are wings rather than arms or front legs. Like mammals, and unlike amphibians and reptiles, birds are warm-blooded-that is, their body temperature always remains about the same, even if the temperature of their surroundings changes. Unlike most other vertebrates, living birds lack teeth. Instead, they have a hard bill, or beak, which they use in getting food and for self-defense. A number of the earliest birds possessed teeth, but these species no longer exist.
Many birds have great value to people. Such birds as chickens and turkeys provide meat and eggs for food. Some kinds of birds help farmers by eating insects that attack their crops. Others eat farmers' grain and fruit. But in general, birds do much more good than harm.
Since the 1600's, about 80 kinds of birds have died out. People have killed off most of these species by overhunting them and by destroying their environment. Today, most countries have laws to protect birds and help prevent any more kinds from dying out.
This article discusses the importance of birds, their distribution throughout the world, how they live, and how they raise a family. The article also describes bird migration, the bodies of birds, bird study and protection, and the evolution of birds.