Bird. I INTRODUCTION Bird, animal with feathers and wings. Birds are the only

Bird. I INTRODUCTION Bird, animal with feathers and wings. Birds are the only animals with feathers, although some other animals, such as insects and bats, also have wings. Nearly all birds can fly, and even flightless birds, such as ostriches and penguins, evolved from flying ancestors. Birds are members of a group of animals called vertebrates, which possess a spinal column or backbone. Other vertebrates are fish, amphibians, reptiles, and mammals. Many characteristics and behaviors of birds are distinct from all other animals, but there are some similarities. Like mammals, birds have four-chambered hearts and are warm-blooded--having a relatively constant body temperature that enables them to live in a wide variety of environments. Like reptiles, birds develop from embryos in eggs outside of the mother's body. Birds are found worldwide in many habitats. They can fly over some of the highest mountains on earth as well as both of the earth's poles, dive through water to depths of more than 250 m (850 ft), and occupy habitats with the most extreme climates on the planet, including arctic tundra and the Sahara Desert. Certain kinds of seabirds are commonly seen over the open ocean thousands of kilometers from the nearest land, but all birds must come ashore to raise their young. Highly developed animals, birds are sensitive and responsive, colorful and graceful, with habits that excite interest and inquiry. People have long been fascinated by birds, in part because birds are found in great abundance and variety in the same habitats in which humans thrive. And like people, most species of birds are active during daylight hours. Humans find inspiration in birds' capacity for flight and in their musical calls. Humans also find birds useful--their flesh and eggs for food, their feathers for warmth, and their companionship. Perhaps a key basis for our rapport with birds is the similarity of our sensory worlds: Both birds and humans rely more heavily on hearing and color vision than on smell. Birds are useful indicators of the quality of the environment, because the health of bird populations mirrors the health of our environment. The rapid decline in bird populations and the accelerating extinction rates of birds in the world's forests, grasslands, wetlands, and islands are therefore reasons for great concern. II PHYSICAL CHARACTERISTICS Birds vary in size from the tiny bee hummingbird, which measures about 57 mm (about 2.25 in) from beak tip to tail tip and weighs 1.6 g (0.06 oz), to the ostrich, which stands 2.7 m (9 ft) tall and weighs up to 156 kg (345 lb). The heaviest flying bird is the great bustard, which can weigh up to 18 kg (40 lb). A Parts of a Bird's Body All birds are covered with feathers, collectively called plumage, which are specialized structures of the epidermis, or outer layer of skin. The main component of feathers is keratin, a flexible protein that also forms the hair and fingernails of mammals. Feathers provide the strong yet lightweight surface area needed for powered, aerodynamic flight. They also serve as insulation, trapping pockets of air to help birds conserve their body heat. The varied patterns, colors, textures, and shapes of feathers help birds to signal their age, sex, social status, and species identity to one another. Some birds have plumage that blends in with their surroundings to provide camouflage, helping these birds escape notice by their predators. Birds use their beaks to preen their feathers, often making use of oil from a gland at the base of their tails. Preening removes dirt and parasites and keeps feathers waterproof and supple. Because feathers are nonliving structures that cannot repair themselves when worn or broken, they must be renewed periodically. Most adult birds molt--lose and replace their feathers--at least once a year. Bird wings are highly modified forelimbs with a skeletal structure resembling that of arms. Wings may be long or short, round or pointed. The shape of a bird's wings influences its style of flight, which may consist of gliding, soaring, or flapping. Wings are powered by flight muscles, which are the largest muscles in birds that fly. Flight muscles are located in the chest and are attached to the wings by large tendons. The breastbone, a large bone shaped like the keel of a boat, supports the flight muscles. Nearly all birds have a tail, which helps them control the direction in which they fly and also plays a role in landing. The paired flight feathers of the tail, called retrices, extend from the margins of a bird's tail. Smaller feathers called coverts lie on top of the retrices. Tails may be square, rounded, pointed, or forked, depending on the lengths of the retrices and the way they terminate. The shapes of bird tails vary more than the shapes of wings, possibly because tail shape is less critical to flight than wing shape. Many male birds, such as pheasants, have ornamental tails that they use to attract mating partners. Birds have two legs; the lower part of each leg is called the tarsus. Most birds have four toes on each foot, and in many birds, including all songbirds, the first toe, called a hallux, points backwards. Bird toes are adapted in various species for grasping perches, climbing, swimming, capturing prey, and carrying and manipulating food. Instead of heavy jaws with teeth, modern birds have toothless, lightweight jaws, called beaks or bills. Unlike humans or other mammals, birds can move their upper jaws independently of the rest of their heads. This helps them to open their mouths extremely wide. Beaks occur in a wide range of shapes and sizes, depending on the type of food a bird eats. The eyes of birds are large and provide excellent vision. They are protected by three eyelids: an upper lid resembling that of humans, a lower lid that closes when a bird sleeps, and a third lid, called a nictitating membrane, that sweeps across the eye sideways, starting from the side near the beak. This lid is a thin, translucent fold of skin that moistens and cleans the eye and protects it from wind and bright light. The ears of birds are completely internal, with openings placed just behind and below the eyes. In most birds, textured feathers called auriculars form a protective screen that prevents objects from entering the ear. Birds rely on their ears for hearing and also for balance, which is especially critical during flight. Two groups of birds, cave swiftlets and oilbirds, find their way in dark places by echolocation--making clicks or rattle calls and interpreting the returning echoes to obtain clues about their environment. The throats of nearly all birds contain a syrinx (plural, syringes), an organ that is comparable to the voice box of mammals. The syrinx has two membranes that produce sound when they vibrate. Birds classified as songbirds have particularly well-developed syringes. Some songbirds, such as the wood thrush, can control each membrane independently; in this way they can sing two songs at the same time. Birds have well-developed brains, which provide acute sensory perception, keen balance and coordination, and instinctive behavior, along with a surprising degree of intelligence. Parts of the bird brain that are especially developed are the optic lobes, where nerve impulses from the eyes are processed, and the cerebellum, which coordinates muscle actions. The cerebral cortex, the part of the brain responsible for thought in humans, is primitive in birds. However, birds have a hyperstriatum--a forebrain component that mammals lack. This part of the brain helps songbirds to learn their songs, and scientists believe that it may also be the source of bird intelligence. B Physical Adaptations for Flight The internal body parts of all birds, including flightless ones, reflect the evolution of birds as flying creatures. Birds have lightweight skeletons in which many of the major bones are hollow. A unique feature of birds is the furculum, or wishbone, which is comparable to the collarbones of humans, although in birds the left and right portions are fused together. The furculum absorbs the shock of wing motion and acts as a spring to help birds breathe while they fly. Several anatomical adaptations help to reduce weight and concentrate it near the center of gravity. For example, modern birds are toothless, which helps reduce the weight of their beaks, and food grinding is carried out in the muscular gizzard, a part of the stomach located near the body's core. The egg-laying habit of birds enables young to develop outside the body of the female, significantly lightening her load. For further weight reduction, the reproductive organs of birds atrophy, or become greatly reduced in size, outside of the breeding season. C Physiology Flight, especially taking off and landing, requires a huge amount of energy--more than humans need even for running. Taking flight is less demanding for small birds than it is for large ones, but small birds need more energy to stay warm. In keeping with their enormous energy needs, birds have an extremely fast metabolism, which includes the chemical reactions involved in releasing stored energy from food. The high body temperature of birds--40° to 42° C (104° to about 108° F)--provides an environment that supports rapid chemical reactions. To sustain this high-speed metabolism, birds need an abundant supply of oxygen, which combines with food molecules within cells to release energy. The respiratory, or breathing, system of birds is adapted to meet their special needs. Unlike humans, birds have lungs with an opening at each end. New air enters the lungs from one end, and used air goes out the other end. The lungs are connected to a series of air sacs, which facilitate the movement of air. Birds breathe faster than any other animal. For example, a flying pigeon breathes 450 times each minute, whereas a human, when running, might breathe only about 30 times each minute. The circulatory system of birds also functions at high speed. Blood vessels pick up oxygen in the lungs and carry it, along with nutrients and other substances essential to life, to all of a bird's body tissues. In contrast to the human heart, which beats about 160 times per minute when a person runs, a small bird's heart beats between 400 and 1,000 times per minute. The hearts of birds are proportionately larger than the hearts of other animals. Birds that migrate and those that live at high altitudes have larger hearts, relative to their body size, than other birds. III BIRDS IN MOTION The characteristic means of locomotion in birds is flight. However, birds are also variously adapted for movement on land, and some are excellent swimmers and divers. A Flying Like airplanes, birds rely on lift--an upward force that counters gravity--in order to fly. Birds generate lift by pushing down on the air with their wings. This action causes the air, in return, to push the wings up. The shape of wings, which have an upper surface that is slightly convex and a lower surface that is concave, contributes to this effect. To turn, birds often tilt so that one wing is higher than the other. Different wing shapes adapt birds for different styles of flight. The short, rounded wings and strong breast muscles of quail are ideal for short bursts of powered flight. Conversely, the albatross's long narrow wings enable these birds to soar effortlessly over windswept ocean surfaces. The long, broad wings of storks, vultures, and eagles provide excellent lift on rising air currents. Feathers play a crucial role in flight. The wings and tails of birds have specialized flight feathers--the largest and strongest type of feathers--that contribute to lift. Because each of the flight feathers is connected to a muscle, birds can adjust their position individually. As a bird pushes down on the air with its wings, its flight feathers overlap to prevent air from passing through. The same feathers twist open on the upstroke, so that air flows between them and less effort is needed to lift the wings. Feathers also help to minimize drag, a force of resistance that acts on solid bodies moving through air. Contour feathers, which are the most abundant type of feather, fill in and cover angular parts of a bird's body, giving birds a smooth, aerodynamic form. Bird tails are also important to flight. Birds tip their tail feathers in different directions to achieve stability and to help change direction while flying. When soaring, birds spread their tail feathers to obtain more lift. When landing, birds turn their tails downward, so that their tails act like brakes. B Walking, Running, and Climbing Most birds can move their legs alternately to walk and run, and some birds are adept at climbing trees. Birds' agility on land varies widely among different species. The American robin both hops and walks, while the starling usually walks. The ostrich can run as fast as 64 km/h (40 mph). Swifts, however, can neither hop nor run; their weak feet are useful only for clinging to vertical surfaces, such as the walls of caves and houses. Birds that walk in shallow water, such as herons and stilts, have long legs that facilitate wading. Jacanas, which walk on lily pads and mud, have long toes and nails that disperse their weight to help prevent them from sinking. Penguins have stubby legs placed far back from their center of gravity. For this reason, they can walk only with an upright posture and a short-stepping gait. When penguins need to move quickly, they "toboggan" on their bellies, propelling themselves across ice with their wings and feet. C Swimming Many birds are excellent swimmers and divers, including such distantly related types of birds as grebes, loons, ducks, auks, cormorants, penguins, and diving petrels. Most of these birds have webbed or lobed toes that act as paddles, which they use to propel themselves underwater. Others, including auks and penguins, use their wings to propel themselves through the water. Swimming birds have broad, raftlike bodies that provide stability. They have dense feather coverings that hold pockets of air for warmth, but they can compress the air out of these pockets to reduce buoyancy when diving. Many fish-catching birds can dive to great depths, either from the air or from the water's surface. The emperor penguin can plunge to depths of more than 250 m (850 ft) and remain submerged for about 12 minutes. Some ducks, swans, and geese perform an action called dabbling, in which they tip their tails up and reach down with their beaks to forage on the mud beneath shallow water. IV HOW BIRDS LIVE Like other animals, birds must eat, rest, and defend themselves against predators in order to survive. They must also reproduce and raise their young to contribute to the survival of their species. For many bird species, migration is an essential part of survival. Birds have acquired remarkably diverse and effective strategies for achieving these ends. A Feeding Birds spend much of their time feeding and searching for food. Most birds cannot store large reserves of food internally, because the extra weight would prevent them from flying. Small birds need to eat even more frequently than large ones, because they have a greater surface area in proportion to their weight and therefore lose their body heat more quickly. Some extremely small birds, such as hummingbirds, have so little food in reserve that they enter a state resembling hibernation during the night and rely on the warmth of the sun to energize them in the morning. Depending on the species, birds eat insects, fish, meat, seeds, nectar, and fruit. Most birds are either carnivorous, meaning they eat other animals, or herbivorous, meaning they eat plant material. Many birds, including crows and gulls, are omnivorous, eating almost anything. Many herbivorous birds feed protein-rich animal material to their growing young. Some bird species have highly specialized diets, such as the Everglade kite, which feeds exclusively on snails. Two unusual internal organs help birds to process food. The gizzard, which is part of a bird's stomach, has thick muscular walls with hard inner ridges. It is capable of crushing large seeds and even shellfish. Some seed-eating birds swallow small stones so that the gizzard will grind food more efficiently. Birds that feed on nectar and soft fruit have poorly developed gizzards. Most birds have a crop--a saclike extension of the esophagus, the tubular organ through which food passes after leaving the mouth. Some birds store food in their crops and transport it to the place where they sleep. Others use the crop to carry food that they will later regurgitate to their offspring. The bills of birds are modified in ways that help birds obtain and handle food. Nectar-feeders, such as hummingbirds, have long thin bills, which they insert into flowers, and specialized tubular or brushlike tongues, through which they draw up nectar. Meat-eating birds, including hawks, owls, and shrikes, have strong, hooked bills that can tear flesh. Many fish-eating birds, such as merganser ducks, have toothlike ridges on their bills that help them to hold their slippery prey. The thick bills and strong jaw muscles of various finches and sparrows are ideal for crushing seeds. Woodpeckers use their bills as chisels, working into dead or living wood to find insect larvae and excavate nest cavities. At least two species of birds use tools in obtaining food. One is the woodpecker finch, which uses twigs or leaf stalks to extract insects from narrow crevices in trees. The other is the Egyptian vulture, which picks up large stones in its bill and throws them at ostrich eggs to crack them open. B Resting Birds need far less sleep than humans do. Birds probably sleep to relax their muscles and conserve energy but not to refresh their brains. Many seabirds, in particular, sleep very little. For example, the sooty tern, which rarely lands on water, may fly for several years with only brief periods of sleep lasting a few seconds each. Flying is so effortless for the sooty tern and some other seabirds that it takes virtually no energy at all. Most birds are active during the day and sleep at night. Exceptions are birds that hunt at night, such as owls and nightjars. Birds use nests for sleeping only during the breeding season. The rest of the year, birds sleep in shrubs, on tree branches, in holes in trees, and on the bare ground. Most ducks sleep on the water. Many birds stand while they sleep, and some birds sleep while perched on a branch--sometimes using only one foot. These birds are able to avoid falling over because of a muscle arrangement that causes their claws to tighten when they bend their legs to relax. C Reproduction In order to reproduce, birds must find a suitable mate, or mates, as well as the necessary resources--food, water, and nesting materials--for caring for their eggs and raising the hatched young to independence. Most birds mate during a specific season in a particular habitat, although some birds may reproduce in varied places and seasons, provided environmental conditions are suitable. Most birds have monogamous mating patterns, meaning that one male and one female mate exclusively with each other for at least one season. However, some bird species are either polygynous, that is, the males mate with more than one female, or polyandrous, in which case the females mate with more than one male (see Animal Courtship and Mating). Among many types of birds, including some jays, several adults, rather than a single breeding pair, often help to raise the young within an individual nest. Birds rely heavily on their two main senses, vision and hearing, in courtship and breeding. Among most songbirds, including the nightingale and the sky lark, males use song to establish breeding territories and attract mates. In many species, female songbirds may be attracted to males that sing the loudest, longest, or most varied songs. Many birds, including starlings, mimic the sounds of other birds. This may help males to achieve sufficiently varied songs to attract females. Numerous birds rely on visual displays of their feathers to obtain a mating partner. For example, the blue bird of paradise hangs upside down from a tree branch to show off the dazzling feathers of its body and tail. A remarkable courtship strategy is exhibited by male bowerbirds of Australia and New Guinea. These birds attract females by building bowers for shelter, which they decorate with colorful objects such as flower petals, feathers, fruit, and even human-made items such as ribbons and tinfoil. Among some grouse, cotingas, the small wading birds called shorebirds, hummingbirds, and other groups, males gather in areas called leks to attract mates through vocal and visual displays. Females visiting the leks select particularly impressive males, and often only one or a very few males actually mate. Among western grebes, both males and females participate in a dramatic courtship ritual called rushing, in which mating partners lift their upper bodies far above the water and paddle rapidly to race side by side over the water's surface. Although male birds usually court females, there are some types of birds, including the phalaropes, in which females court males. Many birds establish breeding territories, which they defend from rivals of the same species. In areas where suitable nesting habitat is limited, birds may nest in large colonies. An example is the crab plover, which sometimes congregates by the thousands in areas of only about 0.6 hectares (about 1.5 acres). For breeding, most birds build nests, which help them to incubate, or warm, the developing eggs. Nests sometimes offer camouflage from predators and physical protection from the elements. Nests may be elaborate constructions or a mere scrape on the ground. Some birds, including many shorebirds, incubate their eggs without any type of nest at all. The male emperor penguin of icy Antarctica incubates the single egg on top of its feet under a fold of skin. Bird nests range in size from the tiny cups of hummingbirds to the huge stick nests of eagles, which may weigh a ton or more. Some birds, such as the malleefowl of southern Australia, use external heat sources, such as decaying plant material, to incubate their eggs. Many birds, including woodpeckers, use tree cavities for nests. Others, such as cowbirds and cuckoos, are brood parasites; they neither build nests nor care for their young. Instead, females of these species lay their eggs in the nests of birds of other species, so that the eggs are incubated--and the hatchlings raised--by birds other than the hatchlings' true parents. Incubation by one or both parents works together with the nest structure to provide an ideal environment for the eggs. The attending parent may warm the eggs with a part of its belly called the brood patch. Bird parents may also wet or shade the eggs to prevent them from overheating. The size, shape, color, and texture of a bird egg is specific to each species. Eggs provide an ideal environment for the developing embryo. The shells of eggs are made from calcium carbonate. They contain thousands of pores through which water can evaporate and air can seep in, enabling the developing embryo to breathe. The number of eggs in a clutch (the egg or eggs laid by a female bird in one nesting effort) may be 15 or more for some birds, including pheasants. In contrast, some large birds, such as condors and albatross, may lay only a single egg every two years. The eggs of many songbirds hatch after developing for as few as ten days, whereas those of albatross and kiwis may require 80 days or more. Among some birds, including songbirds and pelicans, newly hatched young are without feathers, blind, and incapable of regulating their body temperature. Many other birds, such as ducks, are born covered with down and can feed themselves within hours after hatching. Depending on the species, young birds may remain in the nest for as little as part of a day or as long as several months. Fledged young (those that have left the nest) may still rely on parental care for many days or weeks. Only about 10 percent of birds survive their first year of life; the rest die from starvation, disease, predators, or inexperience with the behaviors necessary for survival. The age at which birds begin to breed varies from less than a year in many songbirds and some quail to ten years or more in some albatross. The life spans of birds in the wild are poorly known. Many small songbirds live only three to five years, whereas some albatross are known to have survived more than 60 years in the wild. D Defense The keen eyesight and acute hearing of birds help them react quickly to predators, which may be other birds, such as falcons and hawks, or other types of animals, such as snakes and weasels. Many small birds feed in flocks, where they can benefit from the observing power of numerous pairs of eyes. The first bird in a flock to spot a predator usually warns the others with an alarm call. Birds that feed alone commonly rely on camouflage and rapid flight as means of evading predators. Many birds have highly specific and unusual defense strategies. The burrowing owl in North America, which lives in the burrows of ground squirrels, frightens off predators by making a call that sounds much like a rattlesnake. The snipe, a wading bird, flees from its enemies with a zigzag flight pattern that is hard for other birds to follow. E Migration Many bird species undergo annual migrations, traveling between seasonally productive habitats. Migration helps birds to have continuous sources of food and water, as well as to avoid environments that are too hot or too cold. Some of the most spectacular bird migrations are made by seabirds, which fly across oceans and along coastlines, sometimes traveling 32,000 km (20,000 mi) or more in a single year. Migrating birds use a variety of cues to find their way. These include the positions of the sun during the day and the stars at night; the earth's magnetic field; and visual, olfactory, and auditory landmarks. The strict formations in which many birds fly help them on the journey. For example, migrating geese travel in a V-shaped formation, which enables all of the geese except the leader to take advantage of the updrafts generated by the flapping wings of the goose in front. Young birds of many species undertake their first autumn migration with no guidance from experienced adults. These inexperienced birds do not necessarily reach their destinations; many birds stray in the wrong direction and are sometimes observed thousands of kilometers away from their normal route (see Animal Migration). V TYPES OF BIRDS There are nearly 10,000 known species of modern or recently extinct birds. Traditionally, taxonomists (those who classify living things based on evolutionary relationships) have looked at bird characteristics such as skeletal structure, plumage, and bill shape to determine which birds have a shared evolutionary history. More recently, scientists have turned to deoxyribonucleic acid (DNA)--the genetic information found in the cells of all living organisms--for clues about relationships among birds. DNA is useful to bird taxonomists because closely related birds have more similar DNA than do groups of birds that are distantly related. DNA comparisons have challenged some of scientists' previous ideas about relationships among birds. For example, these studies have revealed that vultures of the Americas are more closely related to storks than to the vultures of Europe, Asia, or Africa. Another method of categorizing birds focuses on adaptive types, or lifestyles. This system groups together birds that live in similar environments or have similar methods for obtaining food. Even among a given adaptive type, birds show tremendous diversity. Some of the widespread adaptive types of birds are discussed below. A Aquatic Birds Aquatic birds obtain most or all of their food from the water. All aquatic birds that live in saltwater environments have salt glands, which enable them to drink seawater and excrete the excess salt. Albatross, shearwaters, storm petrels, and diving petrels are considered the most exclusively marine of all birds. These birds spend much of their time over the open ocean, well away from land. Many other birds have aquatic lifestyles but live closer to land. Among these are penguins, which live in the southernmost oceans near the Antarctic. Some species of penguins spend most of their lives in the water, coming on land only to reproduce and molt. Grebes and divers, or loons, are found on or near lakes. Grebes are unusual among birds because they make their nests on the water, using floating plant materials that they hide among reeds. Pelicans, known for their long bills and huge throat pouches, often switch between salt water and fresh water habitats during the year. Gulls are generalists among the aquatic birds, feeding largely by scavenging over open water, along shores, or even inland areas. Waterfowl, a group that includes ducks, geese, and swans, often breed on freshwater lakes and marshes, although they sometimes make their homes in marine habitats. B Wading Birds Many long-legged, long-billed birds are adapted to live at the junction of land and water. Large wading birds, including herons, storks, ibises, spoonbills, and flamingoes, are found throughout the world, except near the poles. These birds wade in shallow water or across mudflats, wet fields, or similar environments to find food. Depending on the species, large wading birds may eat fish, frogs, shrimp, or microscopic marine life. Many of the large wading birds gather in enormous groups to feed, sleep, or nest. Shorebirds often inhabit puddles or other shallow bodies of water. The diversity of shorebirds is reflected in their varied bill shapes and leg lengths. The smallest North American shorebirds, called stints or peeps, have short, thin bills that enable them to pick at surface prey, whereas curlews probe with their long bills for burrowing shellfish and marine worms that are beyond the reach of most other shore feeders. Avocets and stilts have long legs and long bills, both of which help them to feed in deeper water. C Birds of Prey Among the best-known birds are the birds of prey. Some, including hawks, eagles, and falcons, are active during the daytime. Others, notably owls, are nocturnal, or active at night. Birds of prey have hooked beaks, strong talons or claws on their feet, and keen eyesight and hearing. The larger hawks and eagles prey on small mammals, such as rodents and other vertebrates. Some birds of prey, such as the osprey and many eagles, eat fish. Falcons eat mainly insects, and owls, depending on the species, have diets ranging from insects to fish and mammals. Scavengers that feed on dead animals are also considered birds of prey. These include relatives of eagles called Old World vultures, which live in Eurasia and Africa, and the condors and vultures of North and South America. D Running Birds Some birds, including the largest of all living birds, have lost the ability to fly. The ostriches and their relatives--rheas, emus, cassowaries, and kiwis--are flightless birds found in Africa, South America, and Australia, including New Guinea and New Zealand. The tinamous of Central and South America are related to the ostrich group, but they have a limited ability to fly. Other birds that feed primarily on the ground and are excellent runners include the bustards (relatives of the cranes) and megapodes, members of a group of chickenlike birds that includes quail, turkeys, pheasants, and grouse. Vegetation is an important part of the diets of running birds. E Perching Birds More than half of all living species of birds are perching birds. Perching birds have been successful in all terrestrial habitats. Typically small birds, perching birds have a distinctive arrangement of toes and leg tendons that enables them to perch acrobatically on small twigs. They have the most well-developed and complex vocalizations of all birds. They are divided into two main groups: the sub-oscines, which are mainly tropical and include tyrant flycatchers, antbirds, and ovenbirds, and the oscines or songbirds, which make up about 80 percent of all perching bird species, among them the familiar sparrows, finches, warblers, crows, blackbirds, thrushes, and swallows. Some birds of this group catch and feed upon flying insects. An example is the swallow, which opens its mouth in a large traplike gape to gather food. One specialized group, the dippers, is aquatic; its members obtain their food during short dives in streams and rivers. F Other Land Birds Many other groups of birds thrive in terrestrial habitats. Parrots, known for their brilliantly colored plumage, form a distinctive group of tropical and southern temperate birds that inhabit woodlands and grasslands. Doves and pigeons, like parrots, are seed and fruit eaters but are more widespread and typically more subdued in color. The cuckoos--including the tree-dwelling species such as the European cuckoo, whose call is mimicked by the cuckoo clock, and ground-inhabiting species, such as roadrunners--are land birds. Hummingbirds are a group of nectar- and insect-feeding land birds whose range extends from Alaska to the tip of South America. Woodpeckers and their relatives thrive in forests. Kingfishers are considered land birds despite their habit of eating fish. VI HABITAT AND RANGE OF BIRDS Although birds collectively occupy most of the earth's surface, most individual species are found only in particular regions and habitats. Some species are quite restricted, occurring only on a single oceanic island or an isolated mountaintop, whereas others are cosmopolitan, living in suitable habitats on most continents. The greatest species diversity occurs in the tropics in North and South America, extending from Mexico to South America. This part of the world is especially rich in tyrant flycatchers, ovenbirds, antbirds, tanagers, and hummingbirds. The Australia and New Guinea region has perhaps the most unique groups of birds, because its birds have long been isolated from those of the rest of the world. Emus, cassowaries, and several songbird groups, including birds-of-paradise, are found nowhere else. Africa is the unique home to many bird families, including turacos, secretary birds, and helmet-shrikes. Areas that are further from the equator have less diverse birds. For example, about 225 bird species breed in the British Isles--about half the number of breeding species that inhabit a single reserve in Ecuador or Peru. Despite the abundance of seabirds at its fringes, Antarctica is the poorest bird continent, with only about 20 species. The habitats occupied by birds are also diverse. Tropical rain forests have high species diversity, as do savannas and wetlands. Fewer species generally occupy extremely arid habitats and very high elevations. A given species might be a habitat specialist, such as the marsh wren, which lives only in marshes of cattails or tules, or a generalist, such as the house sparrow, which can thrive in a variety of environments. Many habitats are only seasonally productive for birds. The arctic tundra, for example, teams with birds during the short summer season, when food and water are plentiful. In the winter, however, this habitat is too cold and dry for all but a few species. Many bird species respond to such seasonal changes by undergoing annual migrations. Large numbers of bird species that breed in the United States and Canada move south to winter in Central or northern South America. Similar migrations from temperate regions to tropical ones exist between Europe and Africa, northeastern Asia and southeast Asia and India and, to a lesser degree, from southern Africa and southern South America to the equatorial parts of those continents. VII EVOLUTION OF BIRDS Scientists disagree about many aspects of the evolution of birds. Many paleontologists (scientists who study fossils to learn about prehistoric life) believe that birds evolved from small, predatory dinosaurs called theropods. These scientists say that many skeletal features of birds, such as light, hollow bones and a furculum, were present in theropod dinosaurs prior to the evolution of birds. Others, however, think that birds evolved from an earlier type of reptile called thecodonts--a group that ultimately gave rise to dinosaurs, crocodiles, and the flying reptiles known as pterosaurs. These scientists assert that similarities between birds and theropod dinosaurs are due to a phenomenon called convergent evolution--the evolution of similar traits among groups of organisms that are not necessarily related. Scientists also disagree about how flight evolved. Some scientists believe that flight first occurred when the ancestors of birds climbed trees and glided down from branches. Others theorize that bird flight began from the ground up, when dinosaurs or reptiles ran along the ground and leaped into the air to catch insects or to avoid predators. Continued discovery and analysis of fossils will help clarify the origins of birds. Despite uncertainties about bird evolution, scientists do know that numerous types of birds lived during the Cretaceous Period, which dates to about 138 million to 65 million years ago. Among these birds were Ichthyornis victor, which resembled a gull and had vertebrae similar to those of a fish, and Hesperonis regalis, which was nearly wingless and had vertebrae like those of today's birds. Most birds of the Cretaceous Period are thought to have died out in the mass extinctions--deaths of large numbers of animal species--that took place at the end of the Cretaceous Period. The Tertiary Period directly following the Cretaceous witnessed an explosive evolution of birds. One bird that lived during the Tertiary Period was Diatryma, which stood 1.8 to 2.4 m (about 6 to 8 ft) tall and had massive legs, a huge bill, and very small, underdeveloped wings. Most modern families of birds can be traced back in the fossil record to the early or mid-Eocene Epoch--a stage within the Tertiary Period that occurred about 50 million years ago. Perching birds, called passerines, experienced a tremendous growth in species diversity in the latter part of the Tertiary; today this group is the most diverse order of birds. During the Pleistocene Epoch, from 1.6 million to 10,000 years ago, also known as the Ice Age, glacier ice spread over more than one-fourth of the land surfaces of the earth. These glaciers isolated many groups of birds from other groups with which they had previously interbred. Scientists have long assumed that the resulting isolated breeding groups evolved into the species of birds that exist today. This assumption has been modified as a result of studies involving bird DNA within cellular components called mitochondria. Pairs of species that only recently diverged from a shared ancestry are expected to have more similar mitochondrial DNA than are pairs that diverged in the more distant past. Because mutations in mitochondrial DNA are thought to occur at a fixed rate, some scientists believe that this DNA can be interpreted as a molecular clock that reveals the approximate amount of time that has elapsed since two species diverged from one another. Studies of North American songbirds based on this approach suggest that only the earliest glaciers of the Pleistocene are likely to have played a role in shaping bird species. The evolution of birds has not ended with the birds that we know today. Some bird species are dying out. In addition, the process of speciation--evolutionary changes that result in new species--continues all the time. VIII BIRDS AND HUMANS Birds have been of ecological and economic importance to humans for thousands of years. Archaeological sites reveal that prehistoric people used many kinds of birds for food, ornamentation, and other cultural purposes. The earliest domesticated bird was probably the domestic fowl or chicken, derived from jungle fowls of Southeast Asia. Domesticated chickens existed even before 3000 BC. Other long-domesticated birds are ducks, geese, turkeys, guinea-fowl, and pigeons. Today the adults, young, and eggs of both wild and domesticated birds provide humans with food. People in many parts of Asia even eat nests that certain swiftlets in southeastern Asia construct out of saliva. Birds give us companionship as pets, assume religious significance in many cultures, and, in the case of hawks and falcons, perform work for us as hunters. People in maritime cultures have learned to monitor seabird flocks to find fish, sometimes even using cormorants to do the fishing. Birds are good indicators of the quality of our environment. In the 19th century, coal miners brought caged canaries with them into the mines, knowing that if the birds stopped singing, dangerous mine gases had escaped into the air and poisoned them. Birds provided a comparable warning to humans in the early 1960s, when the numbers of peregrine falcons in the United Kingdom and raptors in the United States suddenly declined. This decline proved to be caused by organochlorine pesticides, such as DDT, which were accumulating in the birds and causing them to produce eggs with overly fragile shells. This decline in the bird populations alerted humans to the possibility that pesticides can harm people as well. Today certain species of birds are considered to be indicators of the environmental health of their habitats. An example of an indicator bird is the northern spotted owl, which can only reproduce within old growth forests in the Pacific Northwest. Many people enjoy bird-watching. Equipped with binoculars and field guides, they identify birds and their songs, often keeping lists of the various species they have witnessed. Scientists who study birds are known as ornithologists. These experts investigate the anatomy, behavior, evolutionary history, ecology, classification, and species distribution of both domesticated and wild birds. A Birds as Menaces In general, birds pose little direct danger to humans. A few birds, such as the cassowaries of New Guinea and northeastern Australia, are capable of killing humans with their strong legs and bladelike claws, but actual attacks are extremely rare. Many birds become quite aggressive when defending a nest site; humans are routinely attacked, and occasionally killed, by hawks engaging in such defense. Birds pose a greater threat to human health as carriers of diseases. Diseases carried by birds that can affect humans include influenza and psittacosis. Negative impacts by birds on humans are primarily economic. Blackbirds, starlings, sparrows, weavers, crows, parrots, and other birds may seriously deplete crops of fruit and grain. Similarly, fish-eating birds, such as cormorants and herons, may adversely impact aquacultural production. However, the economic benefits of wild birds to humans are well documented. Many birds help humans, especially farmers, by eating insects, weeds, slugs, and rodents. B Endangered Birds Although birds, with some exceptions, are tremendously beneficial to humans, humans have a long history of causing harm to birds. Studies of bone deposits on some Pacific islands, including New Zealand and Polynesia, suggest that early humans hunted many hundreds of bird species to extinction. Island birds have always been particularly susceptible to predation by humans. Because these birds have largely evolved in the absence of land-based predators, they are tame and in many cases flightless. They are therefore easy prey for humans and the animals that accompany them, such as rats. The dodo, a flightless pigeonlike bird on the island of Mauritius in the Indian Ocean, was hunted to extinction by humans in the 1600s. With colonial expansion and the technological advances of the 18th and 19th centuries, humans hunted birds on an unprecedented scale. This time period witnessed the extinction of the great auk, a large flightless seabird of the North Atlantic Ocean that was easily killed by sailors for food and oil. The Carolina parakeet also became extinct in this time period, although the last one of these birds survived in the Cincinnati Zoo until 1918. In the 20th century, a time of explosive growth in human populations, the major threats to birds have been the destruction and modification of their habitats. The relentless clearing of hardwood forests outweighed even relentless hunting as the cause of the extinction of the famous passenger pigeon, whose eastern North American populations may have once numbered in the billions. The fragmentation of habitats into small parcels is also harmful to birds, because it increases their vulnerability to predators and parasites. Habitat fragmentation and reduction particularly affects songbirds that breed in North America in the summer and migrate to Mexico, the Caribbean, Central America, and Colombia for the winter. In North America, these birds suffer from forest fragmentation caused by the construction of roads, housing developments, and shopping malls. In the southern part of their range, songbirds are losing traditional nesting sites as tropical forests are destroyed and shade trees are removed from coffee plantations. Pesticides, pollution, and other poisons also threaten today's birds. These substances may kill birds outright, limit their ability to reproduce, or diminish their food supplies. Oil spills have killed thousands of aquatic birds, because birds with oil-drenched feathers cannot fly, float, or stay warm. Acid rain, caused by chemical reactions between airborne pollutants and water and oxygen present in the atmosphere, has decreased the food supply of many birds that feed on fish or other aquatic life in polluted lakes. Many birds are thought to be harmed by selenium, mercury, and other toxic elements present in agricultural runoff and in drainage from mines and power plants. For example, loons in the state of Maine may be in danger due to mercury that drifts into the state from unregulated coal-fired power plants in the Midwest and other sources. Global warming, an increase in the earth's temperature due to a buildup of greenhouse gases, is another potential threat to birds. Many laws have been enacted to protect birds. More than 120 countries signed the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which went into effect in 1975. This treaty regulates or restricts the trade of endangered birds, including many parrots. Treaties between the United States, Canada, and Mexico protect all migratory birds native to North America. In the United States, the Endangered Species Act promotes species and habitat protection. Other means of protecting birds include the creation of sanctuaries and captive breeding programs. Sanctuaries for birds exist all over the world--two examples are the Bharatpur Bird Sanctuary in India's Keoladeo National Park, which protects painted storks, gray herons, and many other bird species; and the National Wildlife Refuge system of the United States. In North America, some endangered birds are bred in settings such as zoos and specialized animal clinics and later released into the wild. Such breeding programs have added significantly to the numbers of whooping cranes, peregrine falcons, and California condors. Many countries, including Costa Rica, are finding they can reap economic benefits, including the promotion of tourism, by protecting the habitats of birds and other wildlife. The protection of the earth's birds will require more than a single strategy. Many endangered birds need a combination of legal protections, habitat management, and control of predators and competitors. Ultimately, humans must decide that the bird's world is worth preserving along with our own. Scientific classification: All birds belong to the class Aves, which is subdivided into 27 orders. Contributed By: Kimball L. Garrett Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.