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Birds are vertebrate animals in the class Aves. There are approximately 8,800 species of birds, divided among 28 living orders. Of these, slightly more than 900 species are found in North America. There has been considerable disagreement among ornithologists about the appropriate level for differentiating species, leading to multiple classification schemes. But however one distinguishes between species, each species belongs to a larger group, called a genus; each genus belongs to a family; and each family belongs to an order. One order, the Passeriformes or perching birds, accounts for more than one-half of the living species of birds.

Birds are believed to have evolved from saurischian dinosaurs, about 150 million years ago. The first truly bird-like animal was Archaeopteryx lithographica, which lived during the Jurassic period, about 130 million years ago. This 3 ft (1 m) long animal is considered to be an evolutionary link between the birds and the dinosaurs. Archaeopteryx had teeth and other dinosaurian characters, but it also had a feathered body and could fly.

Birds can often be characterized by their habitats. Species are more likely to be found in a single habitat in the tropics, than in more temperate regions (with variable climates) where they may have to accommodate several environments. During times of migration, birds may show up in very non-traditional places. Major habitats that serve as homes to birds include polar regions; tundra; alpine regions; coniferous forests; deciduous forests; tropical rainforests; grasslands; deserts; freshwater lakes, ponds, and streams; shore and marshes; and seas.

Most species of birds will defend their territories, particularly in nesting season, against other birds that happen to stray into the area, and are perceived to be a threat to the nest. Territories generally fall into the following categories: mating, nesting, and feeding; mating and nesting; mating; narrowly restricted nesting; feeding; wintering; roosting; and group territory.

Some birds prefer to live apart from other birds, while others are more social. Certain mated woodpeckers, for example, seem to constantly resent each other, and go out of their way to avoid each other's presence. The Australian wood swallows, on the other hand, feed, bathe, roost, attack predators, breed, preen, perform aerial acrobatics, and feed each other in a communal fashion. Assembly into a flock (consisting of either a single species or several different species) offers the advantage of more eyes and ears for spotting predators and finding new food sources.

Most birds have distinctive calls. Males may sing to warn off rivals, or to attract a mate. Some species have separate calls for courtship and for communication. Bird calls serve several functions including: reproductive functions (to proclaim the sex of an individual, advertise for a mate, or establish territorial sovereignty); social functions (space out birds in a given environment, teach the young their species' song, or convey information about food and enemies); or individual functions (provide emotional release, identify individuals to each other, or simply to perfect a song).

All birds have feathers, and in this way are unlike all other types of animals. Most birds loose their old feathers each year, these being replaced by new ones (molting). Usually, molting occurs when the bird is neither nesting nor migrating. Some birds will undergo a complete molt in the summer or fall, and then a partial molt in the spring (replacing feathers of the body and head). The chief functions of feathers are to protect the body and to promote flight.

Because birds have four limbs, they are referred to as tetrapods. However, the fore limbs are highly modified into structures known as wings, which are primarily used for flying or gliding. The hind limbs are mostly used for walking or hopping, so these animals are bipedal when moving on the ground. Some species of birds are flightless, but most can fly or soar well.

The bones of all flying birds are modified for flight, and are relatively light with many hollow regions. The flying birds have a strongly keeled breastbone or sternum, to which the flight muscles are attached. Their pelvic bones are fused into a structure known as a synsacrum. Birds have a relatively long neck and their mandibles are modified into a keratinous beak. They do not have teeth. Birds have a four-chambered heart and a double circulation of the blood, with complete separation of oxygenated and de-oxygenated blood.

Approximately 90% of all birds are monogamous, which is to say that one male bonds with one female. Bonds may last for one nesting period (e.g., house wrens), an entire breeding season (most species), several seasons (e.g., American robins), or life (e.g., geese, swans, hawks, albatrosses, and loons). Current thinking is that most monogamous birds probably mate outside of the primary pair bond, but the pair contributes substantially only to the raising of young in their own nest. Most birds whose young are reared in a nest form monogamous pairs. But monogamous versus polygamous relationships are not rigidly fixed in some species. Swans, hawks, doves, finches, and thrushes, for example, normally show monogamous behavior but occasionally form polygamous bonds.

Situations in which one male mates with more than one female, while each female mates with only one male, are referred to as polygyny. Scientists have noted that when male birds hold territories that vary greatly in the quality of resources, females tend to choose males with high-quality territories as mates. Given the choice between a poor nesting territory and becoming the second mate of a male with a high quality territory, many females will opt for the latter course. In other cases, where territoriality does not appear to be a factor, an overabundance of resources may also lead to polygyny. In North America, red-winged blackbirds and yellow-headed blackbirds sometimes form polygynous relationships.

There are also rare mating systems in which one female mates with more than one male, while each male mates with only one female (known as polyandry). Two forms of polyandry are known. In one case, the female holds a large territory that includes the nesting territories of two or more males who care for the eggs and the young, such as the spotted sandpipers and red phalaropes of North America. In an even rarer type of polyandry, more than one male may mate with a single female with the resultant offspring reared collectively by the female and her mates. The North American acorn woodpecker occasionally exhibits this type of cooperative polyandry.

Courtship behavior may include dances, feeding rituals, flights, posturing, and cries. Frequently, courtship displays highlight some distinctive feature of the male bird's plumage. In addition to singing, male birds may vibrate their wings, fluff their body feathers, raise their bills, thrust their head forwards, or run by taking short steps. But other birds may advertise their skills rather than their coloring to attract a mate. In its courtship behavior, for example, the male tern finds it sufficient to display a freshly caught fish to a potential mate.

Female birds are fertilized internally. They are oviparous, laying relatively large, hard-shelled eggs, with a discrete yolk. The eggs are incubated by one or both parents. The young of almost all species of birds are cared for by their parents.

Birds tend to build nests to cradle their eggs, rather than as residences. (Compared with nesting behavior, the night-time roosting behavior of most birds is poorly understood.) The construction of nests can range from very basic to highly complex. Some birds will use a nest repeatedly, while others will build a new nest for each new brood. Although each type of bird typically has a preferred site for building its nest, some birds may be very accommodating in their choice of nesting sites. It is usually the female that builds the nest, though the males of some species may assist. The materials used in the building of a nest may be of animal, vegetable, or mineral origin, but plant materials are most commonly used.

The number of eggs laid by the female varies widely from species to species. This number may depend on the temperature of the environment, the time in the season, or the size of the food supply. For the most part, only birds incubate eggs by sitting on them and keeping them warm. During the brooding period, some birds may develop a brooding patch on their bellies where feathers and extra blood vessels develop to keep the eggs warm. The number of broods raised in a single year depends on the length of the available season, and the time it takes for one brood to reach independence. There is some evidence that birds living under crowded conditions are less likely to lay extra clutches in a given year.

Upon hatching, young birds may be active and wide-eyed (precocial), or blind and immobile (altricial). Some precocial birds are able to find their own food immediately after being hatched. Most altricial young remain in the nest for several weeks after they learn to leave it. Among precocial species, young birds may be completely independent of their parents, as in the megapodes; or they may follow their parents but find their own food (e.g., ducks and shorebirds). Others may follow their parents, who point out food to them, for example, quail and chickens. Still others, like grebes and rails, may follow their parents and be fed by them.

The young of semi-precocial species are active and wide-eyed at birth. They remain in the nest even though they are able to walk, so they can be fed by their parents (e.g., gulls and terns). Semi-altricial young are unable to leave the nest, and may be born with their eyes open (e.g., herons and hawks), or with their eyes closed (e.g., owls). The passerines are examples of altricial species.

Although few birds are completely vegetarian, most eat plant material. Some birds are seed-eaters, others feed on fruit and berries, and still others feed on buds and green shoots of plants. Waterfowl may eat various parts of aquatic plants and aquatic creatures such as marine worms. But, without insects and other arthropods such as spiders, most birds would starve. Some birds may employ a variety of foraging methods to find their food, while others may have a much narrower range of techniques. In addition to food, birds must have water, which they either drink or obtain from the foods they eat.

Birds require large and dependable sources of rich food to sustain their high metabolisms. Although some birds are able to sustain themselves by only moving a few miles from where they were hatched, most birds, at least in the temperate zones, move from place to place with the change in seasons. Geese and cranes appear to learn their migration routes by accompanying their elders, but other birds, including many songbirds, seem to reach their wintering grounds by pure instinct. Clues to migratory routes may be provided by the position of the sun, the positions of the stars, the earth's magnetic field, and the sound of the ocean crashing against the shore.

Man's fascination with birds dates back thousands of years. Old World cave drawings show ostriches, auks, grouse, passerines, snowy owls, swans, ducks, eagles, and others. Men were even painted with birds' heads. Ancient myths suggest that early man viewed birds not only as food sources, but as colorful mystical creatures capable of flight and disappearance, and the gift of song.

Although there is little doubt that most birds are beneficial, or at least not harmful, to the interests of man, they are frequently perceived as pests. While it is true, birds have wreaked economic havoc on some of man's agricultural activities, it should not be forgotten that birds also feed on insects, rodents, and other life forms that destroy crops.

Both intentionally and unintentionally, man has destroyed many bird species. Two hundred years ago, birds were considered such an inexhaustible resource that the wholesale slaughter of a species, leading to its extinction, hardly raised an eyebrow. But the greatest impact man has had on birds occurred through his expansion into their natural habitats with the construction of farms, cities, roads, and industrial buildings. A by-product of industrial development has been widespread environmental pollution, including pesticides and other noxious species. Intended to rid fields of such insects as the fire ant, pesticides have accumulated in many places traditionally frequented by birds, where they have been subsequently ingested by them. Oil spills have also taken their toll on bird populations. It is not surprising, therefore, that many species have disappeared as a result of man's encroachment on the natural environment. According to one estimate, 85 species of birds, representing 27 families, have become extinct since 1600.

The best approach to limiting man's impact on birds is the simplest one: if man would limit his destruction of the natural environment, many of the habitats of threatened species could be preserved. Interventions that could protect bird populations include checks on unbridled commercial and residential development, and the dumping of environmental pollutants. But for man to adopt these measures would require a fundamental change in his attitude toward the environment, which is paradoxically only likely to occur through his gaining greater first-hand knowledge of nature in its unspoiled state.



Ehrlich, Paul R., David S. Dobkin, and Darryl Wheye. The Birder's Handbook. New York: Simon & Schuster Inc., 1988.

Forshaw, Joseph. Encyclopedia of Birds. New York: Academic Press, 1998.

Mitchell, A. The Guide to Trees of Canada and North America. Surrey, U.K.: Dragon's World Ltd., 1987.

Peterson, Roger Tory. North American Birds. Houghton Mifflin Interactive (CD-ROM). Somerville, MA: Houghton Miflin, 1995.

Sibley, David Allen. The Sibley Guide to Birds. New York: Knopf, 2000.


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—To sit or hatch eggs.


—To periodically shed all or part of a bird's feathers.


—The practice of having only one mate.


—The branch of zoology concerned with the study of birds.


—Producing eggs that hatch outside of the body.


—An order of birds whose members have four toes, three directed forward and one backward, and all joining the foot at the same level.


—Members of the order Passeriformes, which includes perching birds and songbirds such as jays, blackbirds, warblers, and sparrows.


—A mating pattern in which a female mates with more than one male in a single breeding season.


—A mating pattern in which a male mates with more than one female in a single breeding season.

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