From pigeons in big cities to penguins in the remote icy land of Antarctica, all birds have similar characteristics. They all have wings, though they cannot all fly. All birds also have feathers. In fact, that is the one characteristic that makes them unique since birds are the only animals that do have feathers. Where birds liveBirds are found almost everywhere on Earth, from the Antarctic ice caps to tropical forests. There are more than 9,000 known species of birds. Many birds migrate long distances between winter and summer homes in search of a constant food supply. For example, after nesting and raising their young, many European birds travel to Africa for the winter. Physical featuresFlightBirds are able to fly because of their body structure and feathers. A flying bird's body is slender and tapers like a jet airplane. However, not all birds can fly. Penguins spend their time in the water and ostriches have undeveloped wings. Because of this, they must walk or run everywhere. Most birds alternate some walking and swimming with their flying. FeathersA bird's feathers protect it from rain, cold, and heat. Water slides off a bird's feathers without soaking through. Birds fluff out their feathers on cold days. These fluffed feathers hold a layer of warm air next to their skin. In hot weather birds flatten their feathers close to their body. This keeps their skin cool by preventing hot air from reaching it. The entire covering of feathers on a bird is called plumage. A bird's feathers wear out and are replaced in a process called molting. All birds molt at least once a year, usually in the summer or early fall. Birds also preen their feathers. Birds preen by using their bill to clean, rearrange, and apply oils to their feathers. ColoringIn many species of birds the male is more brightly colored than the female. The dull color of the females helps them to remain hidden in their nest. In some cases, the brightly colored male perches a short distance away from the nest. In this way, the male bird draws the attention of enemies to himself and away from the eggs and young. MetabolismLike humans, birds are warm-blooded. This means that they have a constant body temperature that is not affected by the temperature of their surroundings. Their hearts beat faster than human hearts, however, and birds breathe more rapidly than humans. Because of this, birds have a high metabolism, meaning that they use up energy very quickly. Therefore, some small birds must eat almost constantly while they are awake. SensesBirds have very sharp eyesight. An American kestrel hovering 100 feet (30 meters) above a field can spot a grasshopper and drop directly on it. A thin transparent membrane extends across a bird's eyeball. This membrane keeps the eye moist and protects it from dust. A bird's ears are round openings on either side of the head. In general, birds have a narrower range of hearing than human beings. In most birds the sense of smell is not important for survival. SizeBirds vary greatly in size. The bee hummingbird of Cuba is generally accepted as the smallest living bird. It is 2 1/2 inches (6.3 centimeters) long and weighs about 1/10 ounce (less than 3 grams). The largest living bird is the ostrich. Ostriches can stand 8 feet (2.5 meters) tall and weigh 300 pounds (136 kilograms). Other featuresA bird's bones are hollow and very light. Birds also have a beak and a feature called a crop, which is a pouchlike enlargement of the throat. The crop is used to store food. The gizzard is a part of the stomach where digestive juices break down food. The neck of a bird moves more freely than that of any other animal. A flexible neck allows the bird to look out in all directions for approaching danger. The flexible neck also helps the bird catch food and preen its feathers. BehaviorBirds communicate with one another by using many different sounds. For example, some baby chicks stop moving when their mother produces a danger call. A male bird will perch somewhere, such as on a telephone wire or tree branch, and sing loudly to inform other males that a certain territory belongs to him. The same song is also used in courtship to attract the female. Birds keep their feathers clean and smooth. All birds like to bathe. Some even take dry baths in dust, sand, and snow. Such bathing may help get rid of pests in the feathers. Some birds, such as falcons, live alone. Other birds are more social, such as the guanay cormorants of South America that nest in large groups. Feeding habitsMany birds are insect-eaters. They have developed special abilities to catch insects. For example, some birds catch insects by probing for them with their long, sticky tongues. Others dig in the ground with their bills. Birds of prey have claws and hooked bills for feeding on larger animals. Herons have spearlike bills for catching fish. Some birds feed on plant material, such as seeds, fruit, or nectar. Many birds eat a wide variety of foods. ReproductionEach year, many species of birds return to their summer homes, choose their mates, build their nests, lay eggs, and rear their young. The number of eggs in a set varies from 1 to about 20. Nests may be a scrape in the sand or an elaborately woven hanging structure. A type of bird called the swift uses its saliva to hold together branches when making its nest. Most birds sit on their eggs to hatch them. This process, called incubation, takes from 11 to 80 days. The length of time that parents care for young birds varies widely. Some young are capable of finding food for themselves almost immediately. Others depend entirely on their parents for food. Birds and human beingsHumans have interacted with birds in various ways throughout history. In the mythology and literature of many cultures, birds are often featured as characters. Many people enjoy keeping birds as pets, and zoos in many cities import birds from different lands for people to see in person and learn about. Humans have also used birds for their own purposes. For instance, pigeons have long been bred and trained for carrying messages. Birds such as chickens and turkeys are often raised for their meat, and the eggs of certain birds are a source of food for many people. Birds have also been valued for their feathers, which have been used for decoration since early times. Their use in the headdresses of Native Americans is well known. Ostrich farms have been created to produce the soft and fluffy ostrich feathers that are used to make fans. Large quills were once widely used for writing, and feathers have often been used on arrows. Humans can have a negative impact on the bird population because of this interest. In the past many birds were killed for their feathers and quills. In addition, bird habitats are destroyed throughout the world when people cut down trees to create farmland, houses, and other buildings. Pollution also destroys bird habitats. When a habitat is destroyed, the birds are forced to look for a new place to live. Many birds do not survive this move, and in some cases, entire species die out. |