Bird
I
INTRODUCTION
Bird, common name for any member of the class of vertebrates that contains animals with feathers. All adult birds have feathers, although some species, such as pelicans, kingfishers, woodpeckers, and jays, are completely naked when hatched.
Birds share certain features with mammals, such as warm-bloodedness and a four-chambered heart. Nevertheless, birds are distinct, having evolved from dinosaurs long after the mammalian and reptilian groups diverged. All birds—like most reptiles and a few primitive mammals—develop from embryos in eggs outside the mother's body. Unlike most reptile eggs, those of birds have hard shells, which are very strong in large birds and rather brittle in small birds.
II
ANATOMY AND PHYSIOLOGY
Most birds can fly, and all are ultimately descended from ancestors that could fly, although there are many extinct flightless species. The bodies of birds are therefore highly modified for efficiency in flight. The digital and wrist bones of the forelimb are extensively fused to form a rigid support for the large flight feathers of the wing. Fusing of bones for strength and lightness is also found in the skull and pelvic girdle. Many bones of adult birds are hollow rather than filled with marrow, and are connected to a system of air sacs dispersed through the body. The sternum, or breast bone, of most birds is relatively large and bears a central ridge, or keel, known as the carina. The sternum and carina support some of the major muscles used in flying. Flightless (ratite) birds—including the ostrich, the kiwi, and their relatives—do not have a carina and the sternum is reduced in size. The wing structure of most flying birds is the same but there are differences in flying styles. Larger flying birds, such as buzzards, vultures, albatrosses and eagles, spend much of their time in the air gliding or soaring. They use their broad wings with splayed feathers to glide on strong winds or “rest” on rising warm air. Most smaller birds have to flap their wings to sustain their flight.
The jaws of modern birds are extended as toothless bills or beaks, covered with a layer called the ramphotheca, which can be hard, as in most birds, or leathery, as in ducks. The lack of teeth lessens the weight of the skull.
Birds have no sweat glands and cannot cool the body by perspiring. When birds are in flight, excess heat is dispersed by the passage of air through the air sacs. When they are at rest, they rid themselves of heat by panting. The digestive and excretory systems of birds are similar to those of reptiles. The cloaca at the base of a bird's body is used for the excretion of waste products and as the genital opening. Birds produce almost solid urine, uric acid. In sea birds excess salt is excreted in a salt-rich fluid from the nostrils or mouth. It is processed not by the kidney but by a modified tear gland. Other significant internal organs important to birds include the gizzard, which breaks down food, and the crop, which is a chamber that stores food.
A winter survival technique well known in mammals but rare in birds is a slowing of the physiological processes, including reduction of body temperature, resulting in extreme cases in hibernation. Hibernation in birds was long thought to be a myth. Recent research has shown, however, that several species of nightjars, swifts, and hummingbirds that live in deserts or high mountain areas, where winter nights are very cold, can enter a hibernation-like condition of torpor to conserve energy.
III
EVOLUTIONARY HISTORY
The earliest known fossils identified as birds link their ancestry to reptiles, possibly small dinosaurs of the Triassic period (245 million to 208 million years ago). The earliest known fossil bird is Archaeopteryx, about the size of a small pigeon, of which six complete or partial specimens—and an isolated feather—have been found in the Solnhofen limestone beds in Germany; all date from the Late Jurassic period (157 million to 145 million years ago). The species is a mixture of bird-like and dinosaurian anatomical characteristics. If the original skeletons had not clearly shown imprints of feathers exactly like those of modern birds, the fossils might have been classified as small, somewhat peculiar dinosaurs. Archaeopteryx had teeth, which are lacking in all modern birds. Feathers undoubtedly evolved from reptilian scales, but lacking fossil evidence, the mode of transition from scale to feather remains a subject for conjecture. Some skeletal features of this earliest known bird are typical of modern birds, however, and are not known from reptiles.
Fossil bones discovered in Triassic rocks in Texas, United States, in the mid-1980s and named Protoavis texensis in 1991, have been described as a species of bird, 75 million years older than Archaeopteryx, but anatomically closer to modern birds than Archaeopteryx, which lived 150 million years ago. However, the fossil remains consist of isolated bones, not skeletons, and most experts feel that they do not provide sufficient evidence to show that Protoavis was a bird.
Several intermediates between the earliest known bird fossils and modern birds are known. In 1988, fossils in Spain were reported with bird-like tail and shoulder bones but a primitive pelvis and hind limbs. The fossils date from the Early Cretaceous period, about 130 to 120 million years ago. The discovery of another anatomically intermediate fossil was announced in 1990. It is a sparrow-size bird from Liaoning province, China, and is probably only 10 to 15 million years younger than the earliest known bird. Its wings and tail were more like those of modern birds, but its ribs, pelvis, and hind limbs were still primitive. The next good fossils date from about 88 million years ago. These were true birds, little different skeletally from modern birds, except that at least some still bore teeth and longer tails. Most of the species discovered were water birds, largely because conditions for preserving fossils are best in deposits of sand or silt under shallow water. They included a giant diver-like diving bird and one species that perhaps resembled the surviving tern in habits.
Some fossil birds from the end of the Cretaceous period, about 65 million years ago, resembled some living water birds closely enough to be classified with them, but the resemblances are only superficial. The most rapid and diverse evolution of birds took place during the next period, the Tertiary, by the end of which (1.6 million years ago) all groups of living birds had evolved, while some evolutionary lines had died out without leaving descendants.
The Quaternary period, which began about 1.6 million years ago, is divided into two epochs, the Pleistocene and the Holocene (including the present), with the transition usually estimated at about 10,000 years ago. Most living species of birds, or species very much like them, evolved during the Pleistocene. Some Pleistocene species died out completely, possibly because of the severe climatic fluctuations created by the advance and retreat of the great glaciers that have given the Pleistocene epoch the popular name Ice Age.
Extinction is a natural process of evolution, so some species were undoubtedly disappearing when modern human beings appeared. Of the nearly 10,000 species of birds known since historical records began, at least 75 have become extinct. Most of these, such as the dodo and the great auk, were killed off directly by human beings or by animals that people introduced around the world, or else they disappeared because people had altered the environment too severely for the birds to survive. Clearing of forests, draining of swamps and marshes, and other habitat destruction have been so extensive (especially in the Tropics) since the mid-20th century, that it is impossible to estimate how many species of birds have been lost.
IV
CLASSIFICATION
The classification of birds is disputed even among experts. Decisions as to which species are related to one another are usually easy, but at higher levels relationships become more and more uncertain. The relationships of living orders of birds to one another and to orders of birds known from fossils are constantly being argued, especially when new fossils are found or new techniques are discovered for studying modern birds. Early classifications depended entirely on gross anatomy, but these are being re-evaluated with the help of new evidence from such fields as biochemistry, genetics, and comparative behaviour. Anatomical characteristics are being looked at again in an effort to determine which are more primitive and which more advanced. The table of bird orders accompanying this article is only one of several arrangements that have been proposed.
V
DISTRIBUTION
Birds inhabit every continent and almost every island in the world, and are adapted to virtually every ecological environment. Various species live in seemingly sterile deserts, in Antarctica, in jungles, above the tree line on high mountains, in swamps and marshes, on rocky coasts, in woods and fields, and in cities.
Even though most birds are highly mobile because of their power of flight, each species has a definite geographic range, which can encompass several continents or a single tiny island. Two of the most widely distributed species are peregrine falcon and the common barn owl, both of which nest on every continent except Antarctica. In contrast, Semper's warbler is known only from the mountains of the small West Indian island of St Lucia, where it might be approaching extinction. Even entire families have limited distribution. South America, Africa, and Australia each have several bird families that are found nowhere else. Five families are confined to the large island of Madagascar in the Indian Ocean, and four, including the recently extinct moas, are known only from New Zealand. The family with the most limited distribution contains only one species, the kagu—a grey, crested bird the size of a large chicken, found only on the Pacific island of New Caledonia. Only one family, the leafbirds, is restricted to Asia (including adjacent islands to the south and east). No bird families are unique to Europe or North America, although the turkeys (two species of temperate and tropical North America) are often given their own family, instead of being considered a subdivision of the pheasant family.
Several bird families are found around the world in a belt of similar ecological conditions. The divers and auks breed in subarctic and northern temperate regions of North America, Europe, and Asia. Several families—notably the anhingas, parrots, and trogons—inhabit tropical and subtropical parts of North and South America, Africa, and Asia, and the first two have also reached Australia.
VI
ADAPTATIONS
Although all birds share a generally similar body plan, they vary greatly in size and proportions, being adapted to so many ways of life. All of these modifications have to do with survival, including foraging for food, escaping enemies, and protecting eggs and young.
The largest living birds are found among the ratites, all of which have lost the power of flight and have powerful legs for running. Largest of all is the ostrich, standing nearly 2.4 m (almost 8 ft) tall. The smallest are the hummingbirds of the western hemisphere, of which the tiniest is the bee hummingbird of Cuba, only 6.3 cm (2.5 in) from bill tip to tail tip. Its newly hatched young are no larger than honey bees. Hummingbirds are aerial acrobats, being the only birds that regularly fly backwards (to back away from flowers whose nectar they have been lapping).
Many birds pursue prey by swimming under water, but none is so superbly adapted to the task as the penguins. The entire anatomy of the penguin wing has been modified so that it is a stiff, oar-like flipper like that of a porpoise. Clumsy on land, penguins use their wings for underwater propulsion as efficiently as other birds use wings for flying. Most other underwater swimmers—such as divers, grebes, cormorants, and some ducks—are propelled by their powerful feet, although some use their wings for balance. Almost all swimming birds, both the divers and the surface swimmers, have webs of skin connecting their toes, creating efficient paddles. In a few aquatic birds, such as grebes and coots, the toes are not webbed but individually bordered with large flaps or lobes.
Another group of water birds, the tubinares or “tube noses” (named after the shape of their nostrils), consists entirely of marine species: the albatrosses, petrels, and shearwaters. Although they nest on land, usually on islands, they spend most of the year at sea, where they feed on fish and invertebrates. The group includes the greatest size diversity of any order, from sparrow-sized storm petrels to the giant of seabirds, the wandering albatross, with a wingspan reaching up to 3.5 m (11.5 ft).
The group known collectively as birds of prey, or raptors, includes a generally night-hunting order, the owls, and a day-hunting order that includes the hawks, eagles, and falcons, as well as the carrion-feeding vultures. They are all flesh eaters (except for one African vulture that feeds on palm nuts), although the “flesh” for the smaller species is generally insects, and some feed only on fish. All have powerful, sharp bills, and all but the vultures have grasping toes tipped with curved, sharp claws, or talons.
Several families of birds are adapted to feed primarily on flying insects and have developed long wings and wide-opening mouths (although often with small bills). Most highly developed for this way of life are the swifts, whose Latin family name Apodidae means “without feet”. Swifts do have tiny feet, but they are unable even to perch as hummingbirds do; they can only cling to vertical surfaces with their small, sharp claws. The swallows are superficially similar to swifts but are passerine (capable of perching) songbirds, and are not closely related to swifts. The nightjars, or goatsuckers, not only have huge mouths for capturing flying insects but also a row of long hair-like feathers called rictal bristles surrounding the mouth, possibly as a sort of flytrap. Some families of birds, such as the American wood warblers, include species that often catch insects on the wing, and others that seldom do. The fly-catching species have long rictal bristles, whereas among those that pick their insects off leaves or twigs, these bristles are weakly developed, if present at all.
Woodpeckers, which pound on trees not only to excavate their nesting holes but also to communicate with one another by “drumming”, have very thick skulls and a shock-absorber system in their neck muscles and rib cages.
VII
PLUMAGE
The feathers of birds, collectively called plumage, play several roles. Brightly coloured plumage, sometimes including ornamental feathers called plumes, is often influential in attracting a mate, but display of such plumage is used with equal frequency by males to try to intimidate other males competing for females or for territory. Some birds are camouflaged to resemble their surroundings, thus escaping the notice of possible predators (see camouflage). They sometimes even adopt a pose that enhances the protective coloration. The marsh-dwelling herons called bitterns “freeze” with their striped necks and long bills pointing straight up, emphasizing their resemblance to the surrounding reeds. The screech owls, which have a plumage pattern that resembles tree bark, close their large eyes and stretch very thin, thus often passing for a stubby, broken branch. In many species of birds, including most ducks and pheasants, adult males are brightly coloured, whereas the more vulnerable females and young blend into the background. Some birds, notably plovers, have sharply contrasting disruptive camouflage patterns, which break up the outline of the bird when it is standing still, making it quite difficult to spot—the same principle as used by zebras.
Plumage protects all birds against cold, trapping air that acts as insulation. Birds that must endure especially cold winters often have denser plumage than their relatives of more equable climates. The three species of ptarmigan, which are small grouse of Arctic tundras and high mountains, are the only birds that, like some mammals, such as ermines, adopt a nearly pure white coat in winter, making them all but invisible against a background of snow. Swimming birds tend to have hard, water-repellent body feathers, beneath which lies a dense coat of short, fluffy feathers called down. The excellent insulating properties of down, especially from ducks and geese, make it useful as stuffing for winter clothes and sleeping bags.
Most adult birds moult—that is, lose and replace all their feathers—at least once a year, with the exception that moulting of the flight feathers of the wings can extend over two years in a few very large birds such as eagles and cranes. Feathers are subject to physical wear and become faded and brittle with long exposure to sunlight. At the moult, new feathers grow within follicles in the skin, pushing out the old feathers, which are dead structures. Moult cycles often correlate with other cycles. In most migratory species, the new plumage is grown after breeding and before autumn migration. The importance of a bird's feathers is reflected in the amount of time each bird spends preening—cleaning and arranging its plumage with its bill. The preen or uropygeal gland, situated at the base of the bird's tail, secretes oil that is used in preening. The oil keeps the feathers in good condition but also waterproofs them and so is particularly important for sea and water birds.
VIII
SENSES
Most birds have relatively large eyes, especially those that are active in the dim light of dawn and dusk or those that inhabit deep forests. Birds, like human beings, can perceive colours, as might be expected after noting the important role that plumage colour plays in their lives. With few exceptions, birds' eyes are on the sides of their heads rather than in the front as human eyes are. Birds therefore have poor depth perception, but they can see a larger portion of their surroundings without turning their heads. Owls' eyes resemble human eyes in being located on the frontal plane of the head, but they cannot move in their sockets to look from side to side; therefore, owls must turn their faces towards an object to see it. Even owls need a little light to be able to see; those owls that hunt prey in near or total darkness, such as in a cave or old building, use their hearing rather than sight.
Hearing is vitally important to most other birds as well as to owls. Birds communicate among themselves by voice in many ways, often recognizing their mates or young by sound rather than sight. The songs of birds are not always the beautiful sounds of nightingales or canaries, but they are important to all birds, particularly males, in communicating (see Animal Behaviour: Characteristics of Programmed Learning). For the male bird its song will define its territory to other birds. It will also attract a mate. In many species, the more complex the song, the more likely the male bird is to find a mate. The very basic elements of a birdsong are innate but the more elaborate songs are learned. In the wild, young birds recognize, by acoustic sign stimuli, the songs of their own species and commit them to memory. Each bird will not just copy what it has heard but develop a song that has a regional dialect and even an individual voice. A bird reared in captivity, isolated from other birdsongs, will not develop a song beyond the basic elements.
Most birds hear about the same range of sounds as human beings do; some small birds do not hear low sounds but can detect high frequencies that human beings cannot.
The oilbird, a nightjar-like bird of South America, and most of the cave swiftlets of Asia and the Pacific nest deep in caves in total darkness. They manoeuvre by echo location—that is, by making clicking sounds that bounce off the walls of the caves. When the sound returns to the birds' ears, a sonar-like system in their brains indicates the direction and distance of the obstacle. A similar system is found in many bats but is known in no other birds.
The sense of smell is known to be well developed in only a few kinds of birds, but to these few it is very important. In the American vulture family, only the turkey vulture and king vulture have well-developed scent organs; they locate the dead animals on which they feed by using the senses of both smell and sight. The closely related black vultures and condors, as well as the ecologically similar but not closely related Eurasian vultures, have poorly developed scent organs. Petrels, albatrosses, and shearwaters have a strong, oily smell of their own, so it is not surprising that their scent organs are large. The honey guides, small birds of Africa and Asia distantly related to woodpeckers, feed on bee larvae and beeswax, and locate beehives by smell. The kiwis, smallest of the flightless birds, are nearly blind. They locate their food (worms and other invertebrates) by smell and are the only birds that have nostrils at the very tip of the bill.
Little is known about the sense of taste in wild birds, although experiments with chickens and domestic pigeons have shown that they have definite taste preferences.
The sense of touch has been little studied in birds, although they obviously possess it. The eyes of birds are especially sensitive to touch. If anything touches the surface of the eyeball, a third “eyelid” called a nictitating membrane sweeps across the eye, serving to keep it free of bits of dirt or food. The nictitating membrane, which is partly transparent, covers the eyes of swimming or diving birds when they are under water.
Birds have a superb sense of balance and are sensitive to small vibrations. This is vital in maintaining equilibrium on shaky perches as well as in correcting for wind and air currents during flight.
IX
LIFE HISTORY
The life histories of birds are intimately correlated with the seasons. In the Arctic and Temperate zones of both the northern and southern hemispheres, the seasons are the familiar spring, summer, autumn, and winter. In many tropical and subtropical regions, only two seasons are found: rainy and dry—either one rainy and one dry or two of each per year. The onset of rain affects birds in several ways. New vegetation that is used by some birds in nest building appears, and insect populations increase. Temporary lakes and ponds form and are filled with plant and animal foods. For some birds, the dry season is more favourable for nesting and feeding young. Some tropical water birds nest on sandy islands that are revealed only when the water level recedes in large rivers such as the Amazon.
A
Mating and Nesting
Relatively few birds stay with the same mate throughout the year and from one year to another. Even though the same pair may be associated for several years, the relationship between them, called the pair bond, must be renewed or reinforced at the beginning of each breeding season. This is accomplished by displays, which can be visual, auditory, or both. Some visual displays are elaborate and use specialized plumes, as in herons. In other birds, such as ducks, the pair bond is established following a highly stereotyped series of movement patterns. If one of the two birds fails to respond with the correct display, the sequence is broken off. Among the auditory displays is duetting, known in several families such as the wrens and the woodpecker-like barbets. The calls of males and females alternate in such exact succession that the source of the sound may seem to be a single bird. In some species no true pair bond exists. Males display at one another, competing for the right to mate with as many females as possible. Such a gathering of males, called a lek, is found in birds of paradise, wild peafowl (see peacocks) some sandpipers, some grouse, and some members of a tropical family of small birds called manakins.
Eggs are laid in sites varying from bare ground to highly elaborate nests; those of the weaverbirds of Africa and Asia include some of the most intricately constructed objects known in the animal kingdom. Nests are made with a variety of readily available materials: grass, twigs, bark, lichens, plant fibres, feathers, mammal hair, spiders' webs, mud, seaweed, seashells, pebbles, and even the bird's own saliva. Substances such as bits of paper, plastic, and string may also be used. Many birds pluck feathers from their abdomen to line the nest; the exposed skin (known as a brood patch) also helps to warm the egg. The number of eggs per nest varies from only one, in many species, to a dozen or more. In most species the parents take turns incubating the eggs, or the female does it alone. The roles of the sexes are reversed in a few species, with incubation of eggs and brooding of young left to the males. In such species the female is usually larger than the male and more brightly coloured, again the reverse of what is usual.
B
Family Life and Survival
Young birds at hatching fall into two general classes: altricial and precocial. Altricial young are hatched blind and naked, or covered with sparse downy plumage; they cannot support themselves on their legs and are wholly dependent on their parents. Precocial young have their eyes open, are densely covered with down, can walk and run soon after hatching, and can find some of their own food within a few days. Intermediate conditions also exist.
All the songbirds and their close relatives have altricial young, as do such birds as woodpeckers, kingfishers, swifts, and pelicans. Young of the turkey, pheasant, quail, fowl, geese, ducks, and swans are among the most precocial. Among the intermediate are young birds of prey and marine birds, which are relatively helpless but densely covered in down, and young gulls and terns, which are hatched with down and open eyes, and can run within a day or two, but depend on their parents for food for several weeks. Some birds, such as certain cuckoos, honey guides, and widow-birds, are brood parasites and let other birds carry out their parental duties for them. They lay their eggs in the nests of other birds and leave them to be incubated, hatched, and fed by the host parents.
In most birds the family breaks up as soon as the young are fully capable of feeding themselves, and all then go their separate ways. In some large birds, such as swans and cranes, families may stay together during migration and through the winter. Recent studies show that among various species in several orders, young birds may stay with their parents for one to three years, helping to feed and guard the young of successive years before going off to find mates.
For a stable population, births and deaths must balance each other at least approximately. Mortality is always highest among the young, so adults produce more offspring than the number needed to replace themselves. Among migrants, hazards of the journey probably account for most of the loss of young birds. Sedentary tropical land birds encounter relatively more predators than do Temperate Zone birds, losing a higher proportion of eggs and young in this way. They are usually prepared to nest again as often as necessary until a family of young survives. Life expectancy in birds, as in mammals, is roughly correlated with size. Small songbirds may live to 12 years or more, but this is exceptional. Even relatively small seabirds, such as terns, tend to have long lives for their size, actively breeding even after more than 20 years. Longevity in the wild, however, almost never reaches the figures attained by captive birds guarded against disease and predation. Among the longest-living birds in zoos are parrots, large waterfowl, and large birds of prey.
C
Migration
In both arctic and temperate regions, some species of birds are permanent residents, staying in the breeding area all year, although breeding itself is strictly a spring-to-summer phenomenon. Many tropical birds also spend the whole year in the same area; some of these, where seasonality is at a minimum, may nest at almost any time. Most birds of the arctic and temperate regions, and some tropical birds migrate, however—that is, they make regular seasonal movements away from and back to the breeding area. This might be no more than a movement from exposed high mountains down to sheltered valleys for the winter. The opposite extreme is the long-distance migration undertaken annually by many species, the most famous being the Arctic tern, which migrates from the northern latitudes of Eurasia and North America all the way to subantarctic waters.
Long-distance migration raises the intriguing question of how birds find their way. Some fly only by night, others fly over featureless seas; these birds cannot use landmarks as some day migrants seem to do. Scientists now know that no single navigation system exists. Some birds seem to steer by star patterns, and others by the angle of the Sun. At least some birds can detect ultraviolet radiation, the magnetic field of the Earth, and very deep sound vibrations such as those caused by distant ocean waves, but the actual sensory mechanism by which birds translate environmental signals into navigational aids is still a puzzle.
X
BIRDS AND HUMAN BEINGS
Since long before recorded history, as shown by archaeological remains, people have used birds for food. Some tribal peoples still rely heavily on wild birds or their eggs for protein, but with agriculture and civilization came domestication. In most of the world, the birds that are used for human food (mostly chickens, turkeys, guinea fowl, ducks, geese, and pigeons) are bred and raised for that purpose. Eggs are also obtained from domestic birds. Even though hunting for food is no longer necessary in most countries, hunting birds for sport is widespread. Virtually all countries have laws regulating the killing of birds, with most species given protection, and limits are placed on the hunting season and the numbers of game birds that can be taken. Enforcement of such laws, however, is highly variable from country to country.
Birds have played a major role in legends, religious customs, and literature. Feathers have been used for ornamental and ritual purposes by nearly all cultures, and for warm clothing by northern peoples. Birdsongs and calls have inspired tribal religious chants as well as orchestral compositions.
A few species of birds can be detrimental to human interests, principally those that damage fruit and grain crops. Gulls and starlings, attracted to the rubbish dumps that are often near airports, have collided with aircraft, sometimes causing fatal crashes.
By the mid-20th century, the hobby of bird-watching was developing into an industry. People have always enjoyed looking at and listening to birds, but an increasing number have taken an interest in identifying birds and in travelling to new areas to see them. Books and magazines about birds, recordings of birdsong, and binoculars and telescopes for bird-watching are sold in tremendous numbers. Amateurs as well as professionals photograph birds and record their song. Many dozens of travel agencies and individual tour guides specialize in leading bird-watching trips. In common with other forms of tourism, such trips are valuable to the economies of the visited areas and countries as well as to the manufacturers of the bird-watching equipment. Moreover, this enormous popular interest in birds seems to bode well for public support of wildlife conservation measures and for ecological awareness in general. Many amateurs have gone well beyond the “bird-listing” stage and, either on their own or under professional direction, have made important contributions to ornithology (the study of birds).
Scientific classification: Birds make up the class Aves. The earliest known fossil bird is classified as Archaeopteryx lithographica. The fossil bones discovered in Texas were given the name Protoavis. The bird fossils that date back 88 million years include Hesperornis, the giant diver-like diving bird, and Ichthyornis, which might have resembled the living tern in habits.
I
INTRODUCTION
Bird, common name for any member of the class of vertebrates that contains animals with feathers. All adult birds have feathers, although some species, such as pelicans, kingfishers, woodpeckers, and jays, are completely naked when hatched.
Birds share certain features with mammals, such as warm-bloodedness and a four-chambered heart. Nevertheless, birds are distinct, having evolved from dinosaurs long after the mammalian and reptilian groups diverged. All birds—like most reptiles and a few primitive mammals—develop from embryos in eggs outside the mother's body. Unlike most reptile eggs, those of birds have hard shells, which are very strong in large birds and rather brittle in small birds.
II
ANATOMY AND PHYSIOLOGY
Most birds can fly, and all are ultimately descended from ancestors that could fly, although there are many extinct flightless species. The bodies of birds are therefore highly modified for efficiency in flight. The digital and wrist bones of the forelimb are extensively fused to form a rigid support for the large flight feathers of the wing. Fusing of bones for strength and lightness is also found in the skull and pelvic girdle. Many bones of adult birds are hollow rather than filled with marrow, and are connected to a system of air sacs dispersed through the body. The sternum, or breast bone, of most birds is relatively large and bears a central ridge, or keel, known as the carina. The sternum and carina support some of the major muscles used in flying. Flightless (ratite) birds—including the ostrich, the kiwi, and their relatives—do not have a carina and the sternum is reduced in size. The wing structure of most flying birds is the same but there are differences in flying styles. Larger flying birds, such as buzzards, vultures, albatrosses and eagles, spend much of their time in the air gliding or soaring. They use their broad wings with splayed feathers to glide on strong winds or “rest” on rising warm air. Most smaller birds have to flap their wings to sustain their flight.
The jaws of modern birds are extended as toothless bills or beaks, covered with a layer called the ramphotheca, which can be hard, as in most birds, or leathery, as in ducks. The lack of teeth lessens the weight of the skull.
Birds have no sweat glands and cannot cool the body by perspiring. When birds are in flight, excess heat is dispersed by the passage of air through the air sacs. When they are at rest, they rid themselves of heat by panting. The digestive and excretory systems of birds are similar to those of reptiles. The cloaca at the base of a bird's body is used for the excretion of waste products and as the genital opening. Birds produce almost solid urine, uric acid. In sea birds excess salt is excreted in a salt-rich fluid from the nostrils or mouth. It is processed not by the kidney but by a modified tear gland. Other significant internal organs important to birds include the gizzard, which breaks down food, and the crop, which is a chamber that stores food.
A winter survival technique well known in mammals but rare in birds is a slowing of the physiological processes, including reduction of body temperature, resulting in extreme cases in hibernation. Hibernation in birds was long thought to be a myth. Recent research has shown, however, that several species of nightjars, swifts, and hummingbirds that live in deserts or high mountain areas, where winter nights are very cold, can enter a hibernation-like condition of torpor to conserve energy.
III
EVOLUTIONARY HISTORY
The earliest known fossils identified as birds link their ancestry to reptiles, possibly small dinosaurs of the Triassic period (245 million to 208 million years ago). The earliest known fossil bird is Archaeopteryx, about the size of a small pigeon, of which six complete or partial specimens—and an isolated feather—have been found in the Solnhofen limestone beds in Germany; all date from the Late Jurassic period (157 million to 145 million years ago). The species is a mixture of bird-like and dinosaurian anatomical characteristics. If the original skeletons had not clearly shown imprints of feathers exactly like those of modern birds, the fossils might have been classified as small, somewhat peculiar dinosaurs. Archaeopteryx had teeth, which are lacking in all modern birds. Feathers undoubtedly evolved from reptilian scales, but lacking fossil evidence, the mode of transition from scale to feather remains a subject for conjecture. Some skeletal features of this earliest known bird are typical of modern birds, however, and are not known from reptiles.
Fossil bones discovered in Triassic rocks in Texas, United States, in the mid-1980s and named Protoavis texensis in 1991, have been described as a species of bird, 75 million years older than Archaeopteryx, but anatomically closer to modern birds than Archaeopteryx, which lived 150 million years ago. However, the fossil remains consist of isolated bones, not skeletons, and most experts feel that they do not provide sufficient evidence to show that Protoavis was a bird.
Several intermediates between the earliest known bird fossils and modern birds are known. In 1988, fossils in Spain were reported with bird-like tail and shoulder bones but a primitive pelvis and hind limbs. The fossils date from the Early Cretaceous period, about 130 to 120 million years ago. The discovery of another anatomically intermediate fossil was announced in 1990. It is a sparrow-size bird from Liaoning province, China, and is probably only 10 to 15 million years younger than the earliest known bird. Its wings and tail were more like those of modern birds, but its ribs, pelvis, and hind limbs were still primitive. The next good fossils date from about 88 million years ago. These were true birds, little different skeletally from modern birds, except that at least some still bore teeth and longer tails. Most of the species discovered were water birds, largely because conditions for preserving fossils are best in deposits of sand or silt under shallow water. They included a giant diver-like diving bird and one species that perhaps resembled the surviving tern in habits.
Some fossil birds from the end of the Cretaceous period, about 65 million years ago, resembled some living water birds closely enough to be classified with them, but the resemblances are only superficial. The most rapid and diverse evolution of birds took place during the next period, the Tertiary, by the end of which (1.6 million years ago) all groups of living birds had evolved, while some evolutionary lines had died out without leaving descendants.
The Quaternary period, which began about 1.6 million years ago, is divided into two epochs, the Pleistocene and the Holocene (including the present), with the transition usually estimated at about 10,000 years ago. Most living species of birds, or species very much like them, evolved during the Pleistocene. Some Pleistocene species died out completely, possibly because of the severe climatic fluctuations created by the advance and retreat of the great glaciers that have given the Pleistocene epoch the popular name Ice Age.
Extinction is a natural process of evolution, so some species were undoubtedly disappearing when modern human beings appeared. Of the nearly 10,000 species of birds known since historical records began, at least 75 have become extinct. Most of these, such as the dodo and the great auk, were killed off directly by human beings or by animals that people introduced around the world, or else they disappeared because people had altered the environment too severely for the birds to survive. Clearing of forests, draining of swamps and marshes, and other habitat destruction have been so extensive (especially in the Tropics) since the mid-20th century, that it is impossible to estimate how many species of birds have been lost.
IV
CLASSIFICATION
The classification of birds is disputed even among experts. Decisions as to which species are related to one another are usually easy, but at higher levels relationships become more and more uncertain. The relationships of living orders of birds to one another and to orders of birds known from fossils are constantly being argued, especially when new fossils are found or new techniques are discovered for studying modern birds. Early classifications depended entirely on gross anatomy, but these are being re-evaluated with the help of new evidence from such fields as biochemistry, genetics, and comparative behaviour. Anatomical characteristics are being looked at again in an effort to determine which are more primitive and which more advanced. The table of bird orders accompanying this article is only one of several arrangements that have been proposed.
V
DISTRIBUTION
Birds inhabit every continent and almost every island in the world, and are adapted to virtually every ecological environment. Various species live in seemingly sterile deserts, in Antarctica, in jungles, above the tree line on high mountains, in swamps and marshes, on rocky coasts, in woods and fields, and in cities.
Even though most birds are highly mobile because of their power of flight, each species has a definite geographic range, which can encompass several continents or a single tiny island. Two of the most widely distributed species are peregrine falcon and the common barn owl, both of which nest on every continent except Antarctica. In contrast, Semper's warbler is known only from the mountains of the small West Indian island of St Lucia, where it might be approaching extinction. Even entire families have limited distribution. South America, Africa, and Australia each have several bird families that are found nowhere else. Five families are confined to the large island of Madagascar in the Indian Ocean, and four, including the recently extinct moas, are known only from New Zealand. The family with the most limited distribution contains only one species, the kagu—a grey, crested bird the size of a large chicken, found only on the Pacific island of New Caledonia. Only one family, the leafbirds, is restricted to Asia (including adjacent islands to the south and east). No bird families are unique to Europe or North America, although the turkeys (two species of temperate and tropical North America) are often given their own family, instead of being considered a subdivision of the pheasant family.
Several bird families are found around the world in a belt of similar ecological conditions. The divers and auks breed in subarctic and northern temperate regions of North America, Europe, and Asia. Several families—notably the anhingas, parrots, and trogons—inhabit tropical and subtropical parts of North and South America, Africa, and Asia, and the first two have also reached Australia.
VI
ADAPTATIONS
Although all birds share a generally similar body plan, they vary greatly in size and proportions, being adapted to so many ways of life. All of these modifications have to do with survival, including foraging for food, escaping enemies, and protecting eggs and young.
The largest living birds are found among the ratites, all of which have lost the power of flight and have powerful legs for running. Largest of all is the ostrich, standing nearly 2.4 m (almost 8 ft) tall. The smallest are the hummingbirds of the western hemisphere, of which the tiniest is the bee hummingbird of Cuba, only 6.3 cm (2.5 in) from bill tip to tail tip. Its newly hatched young are no larger than honey bees. Hummingbirds are aerial acrobats, being the only birds that regularly fly backwards (to back away from flowers whose nectar they have been lapping).
Many birds pursue prey by swimming under water, but none is so superbly adapted to the task as the penguins. The entire anatomy of the penguin wing has been modified so that it is a stiff, oar-like flipper like that of a porpoise. Clumsy on land, penguins use their wings for underwater propulsion as efficiently as other birds use wings for flying. Most other underwater swimmers—such as divers, grebes, cormorants, and some ducks—are propelled by their powerful feet, although some use their wings for balance. Almost all swimming birds, both the divers and the surface swimmers, have webs of skin connecting their toes, creating efficient paddles. In a few aquatic birds, such as grebes and coots, the toes are not webbed but individually bordered with large flaps or lobes.
Another group of water birds, the tubinares or “tube noses” (named after the shape of their nostrils), consists entirely of marine species: the albatrosses, petrels, and shearwaters. Although they nest on land, usually on islands, they spend most of the year at sea, where they feed on fish and invertebrates. The group includes the greatest size diversity of any order, from sparrow-sized storm petrels to the giant of seabirds, the wandering albatross, with a wingspan reaching up to 3.5 m (11.5 ft).
The group known collectively as birds of prey, or raptors, includes a generally night-hunting order, the owls, and a day-hunting order that includes the hawks, eagles, and falcons, as well as the carrion-feeding vultures. They are all flesh eaters (except for one African vulture that feeds on palm nuts), although the “flesh” for the smaller species is generally insects, and some feed only on fish. All have powerful, sharp bills, and all but the vultures have grasping toes tipped with curved, sharp claws, or talons.
Several families of birds are adapted to feed primarily on flying insects and have developed long wings and wide-opening mouths (although often with small bills). Most highly developed for this way of life are the swifts, whose Latin family name Apodidae means “without feet”. Swifts do have tiny feet, but they are unable even to perch as hummingbirds do; they can only cling to vertical surfaces with their small, sharp claws. The swallows are superficially similar to swifts but are passerine (capable of perching) songbirds, and are not closely related to swifts. The nightjars, or goatsuckers, not only have huge mouths for capturing flying insects but also a row of long hair-like feathers called rictal bristles surrounding the mouth, possibly as a sort of flytrap. Some families of birds, such as the American wood warblers, include species that often catch insects on the wing, and others that seldom do. The fly-catching species have long rictal bristles, whereas among those that pick their insects off leaves or twigs, these bristles are weakly developed, if present at all.
Woodpeckers, which pound on trees not only to excavate their nesting holes but also to communicate with one another by “drumming”, have very thick skulls and a shock-absorber system in their neck muscles and rib cages.
VII
PLUMAGE
The feathers of birds, collectively called plumage, play several roles. Brightly coloured plumage, sometimes including ornamental feathers called plumes, is often influential in attracting a mate, but display of such plumage is used with equal frequency by males to try to intimidate other males competing for females or for territory. Some birds are camouflaged to resemble their surroundings, thus escaping the notice of possible predators (see camouflage). They sometimes even adopt a pose that enhances the protective coloration. The marsh-dwelling herons called bitterns “freeze” with their striped necks and long bills pointing straight up, emphasizing their resemblance to the surrounding reeds. The screech owls, which have a plumage pattern that resembles tree bark, close their large eyes and stretch very thin, thus often passing for a stubby, broken branch. In many species of birds, including most ducks and pheasants, adult males are brightly coloured, whereas the more vulnerable females and young blend into the background. Some birds, notably plovers, have sharply contrasting disruptive camouflage patterns, which break up the outline of the bird when it is standing still, making it quite difficult to spot—the same principle as used by zebras.
Plumage protects all birds against cold, trapping air that acts as insulation. Birds that must endure especially cold winters often have denser plumage than their relatives of more equable climates. The three species of ptarmigan, which are small grouse of Arctic tundras and high mountains, are the only birds that, like some mammals, such as ermines, adopt a nearly pure white coat in winter, making them all but invisible against a background of snow. Swimming birds tend to have hard, water-repellent body feathers, beneath which lies a dense coat of short, fluffy feathers called down. The excellent insulating properties of down, especially from ducks and geese, make it useful as stuffing for winter clothes and sleeping bags.
Most adult birds moult—that is, lose and replace all their feathers—at least once a year, with the exception that moulting of the flight feathers of the wings can extend over two years in a few very large birds such as eagles and cranes. Feathers are subject to physical wear and become faded and brittle with long exposure to sunlight. At the moult, new feathers grow within follicles in the skin, pushing out the old feathers, which are dead structures. Moult cycles often correlate with other cycles. In most migratory species, the new plumage is grown after breeding and before autumn migration. The importance of a bird's feathers is reflected in the amount of time each bird spends preening—cleaning and arranging its plumage with its bill. The preen or uropygeal gland, situated at the base of the bird's tail, secretes oil that is used in preening. The oil keeps the feathers in good condition but also waterproofs them and so is particularly important for sea and water birds.
VIII
SENSES
Most birds have relatively large eyes, especially those that are active in the dim light of dawn and dusk or those that inhabit deep forests. Birds, like human beings, can perceive colours, as might be expected after noting the important role that plumage colour plays in their lives. With few exceptions, birds' eyes are on the sides of their heads rather than in the front as human eyes are. Birds therefore have poor depth perception, but they can see a larger portion of their surroundings without turning their heads. Owls' eyes resemble human eyes in being located on the frontal plane of the head, but they cannot move in their sockets to look from side to side; therefore, owls must turn their faces towards an object to see it. Even owls need a little light to be able to see; those owls that hunt prey in near or total darkness, such as in a cave or old building, use their hearing rather than sight.
Hearing is vitally important to most other birds as well as to owls. Birds communicate among themselves by voice in many ways, often recognizing their mates or young by sound rather than sight. The songs of birds are not always the beautiful sounds of nightingales or canaries, but they are important to all birds, particularly males, in communicating (see Animal Behaviour: Characteristics of Programmed Learning). For the male bird its song will define its territory to other birds. It will also attract a mate. In many species, the more complex the song, the more likely the male bird is to find a mate. The very basic elements of a birdsong are innate but the more elaborate songs are learned. In the wild, young birds recognize, by acoustic sign stimuli, the songs of their own species and commit them to memory. Each bird will not just copy what it has heard but develop a song that has a regional dialect and even an individual voice. A bird reared in captivity, isolated from other birdsongs, will not develop a song beyond the basic elements.
Most birds hear about the same range of sounds as human beings do; some small birds do not hear low sounds but can detect high frequencies that human beings cannot.
The oilbird, a nightjar-like bird of South America, and most of the cave swiftlets of Asia and the Pacific nest deep in caves in total darkness. They manoeuvre by echo location—that is, by making clicking sounds that bounce off the walls of the caves. When the sound returns to the birds' ears, a sonar-like system in their brains indicates the direction and distance of the obstacle. A similar system is found in many bats but is known in no other birds.
The sense of smell is known to be well developed in only a few kinds of birds, but to these few it is very important. In the American vulture family, only the turkey vulture and king vulture have well-developed scent organs; they locate the dead animals on which they feed by using the senses of both smell and sight. The closely related black vultures and condors, as well as the ecologically similar but not closely related Eurasian vultures, have poorly developed scent organs. Petrels, albatrosses, and shearwaters have a strong, oily smell of their own, so it is not surprising that their scent organs are large. The honey guides, small birds of Africa and Asia distantly related to woodpeckers, feed on bee larvae and beeswax, and locate beehives by smell. The kiwis, smallest of the flightless birds, are nearly blind. They locate their food (worms and other invertebrates) by smell and are the only birds that have nostrils at the very tip of the bill.
Little is known about the sense of taste in wild birds, although experiments with chickens and domestic pigeons have shown that they have definite taste preferences.
The sense of touch has been little studied in birds, although they obviously possess it. The eyes of birds are especially sensitive to touch. If anything touches the surface of the eyeball, a third “eyelid” called a nictitating membrane sweeps across the eye, serving to keep it free of bits of dirt or food. The nictitating membrane, which is partly transparent, covers the eyes of swimming or diving birds when they are under water.
Birds have a superb sense of balance and are sensitive to small vibrations. This is vital in maintaining equilibrium on shaky perches as well as in correcting for wind and air currents during flight.
IX
LIFE HISTORY
The life histories of birds are intimately correlated with the seasons. In the Arctic and Temperate zones of both the northern and southern hemispheres, the seasons are the familiar spring, summer, autumn, and winter. In many tropical and subtropical regions, only two seasons are found: rainy and dry—either one rainy and one dry or two of each per year. The onset of rain affects birds in several ways. New vegetation that is used by some birds in nest building appears, and insect populations increase. Temporary lakes and ponds form and are filled with plant and animal foods. For some birds, the dry season is more favourable for nesting and feeding young. Some tropical water birds nest on sandy islands that are revealed only when the water level recedes in large rivers such as the Amazon.
A
Mating and Nesting
Relatively few birds stay with the same mate throughout the year and from one year to another. Even though the same pair may be associated for several years, the relationship between them, called the pair bond, must be renewed or reinforced at the beginning of each breeding season. This is accomplished by displays, which can be visual, auditory, or both. Some visual displays are elaborate and use specialized plumes, as in herons. In other birds, such as ducks, the pair bond is established following a highly stereotyped series of movement patterns. If one of the two birds fails to respond with the correct display, the sequence is broken off. Among the auditory displays is duetting, known in several families such as the wrens and the woodpecker-like barbets. The calls of males and females alternate in such exact succession that the source of the sound may seem to be a single bird. In some species no true pair bond exists. Males display at one another, competing for the right to mate with as many females as possible. Such a gathering of males, called a lek, is found in birds of paradise, wild peafowl (see peacocks) some sandpipers, some grouse, and some members of a tropical family of small birds called manakins.
Eggs are laid in sites varying from bare ground to highly elaborate nests; those of the weaverbirds of Africa and Asia include some of the most intricately constructed objects known in the animal kingdom. Nests are made with a variety of readily available materials: grass, twigs, bark, lichens, plant fibres, feathers, mammal hair, spiders' webs, mud, seaweed, seashells, pebbles, and even the bird's own saliva. Substances such as bits of paper, plastic, and string may also be used. Many birds pluck feathers from their abdomen to line the nest; the exposed skin (known as a brood patch) also helps to warm the egg. The number of eggs per nest varies from only one, in many species, to a dozen or more. In most species the parents take turns incubating the eggs, or the female does it alone. The roles of the sexes are reversed in a few species, with incubation of eggs and brooding of young left to the males. In such species the female is usually larger than the male and more brightly coloured, again the reverse of what is usual.
B
Family Life and Survival
Young birds at hatching fall into two general classes: altricial and precocial. Altricial young are hatched blind and naked, or covered with sparse downy plumage; they cannot support themselves on their legs and are wholly dependent on their parents. Precocial young have their eyes open, are densely covered with down, can walk and run soon after hatching, and can find some of their own food within a few days. Intermediate conditions also exist.
All the songbirds and their close relatives have altricial young, as do such birds as woodpeckers, kingfishers, swifts, and pelicans. Young of the turkey, pheasant, quail, fowl, geese, ducks, and swans are among the most precocial. Among the intermediate are young birds of prey and marine birds, which are relatively helpless but densely covered in down, and young gulls and terns, which are hatched with down and open eyes, and can run within a day or two, but depend on their parents for food for several weeks. Some birds, such as certain cuckoos, honey guides, and widow-birds, are brood parasites and let other birds carry out their parental duties for them. They lay their eggs in the nests of other birds and leave them to be incubated, hatched, and fed by the host parents.
In most birds the family breaks up as soon as the young are fully capable of feeding themselves, and all then go their separate ways. In some large birds, such as swans and cranes, families may stay together during migration and through the winter. Recent studies show that among various species in several orders, young birds may stay with their parents for one to three years, helping to feed and guard the young of successive years before going off to find mates.
For a stable population, births and deaths must balance each other at least approximately. Mortality is always highest among the young, so adults produce more offspring than the number needed to replace themselves. Among migrants, hazards of the journey probably account for most of the loss of young birds. Sedentary tropical land birds encounter relatively more predators than do Temperate Zone birds, losing a higher proportion of eggs and young in this way. They are usually prepared to nest again as often as necessary until a family of young survives. Life expectancy in birds, as in mammals, is roughly correlated with size. Small songbirds may live to 12 years or more, but this is exceptional. Even relatively small seabirds, such as terns, tend to have long lives for their size, actively breeding even after more than 20 years. Longevity in the wild, however, almost never reaches the figures attained by captive birds guarded against disease and predation. Among the longest-living birds in zoos are parrots, large waterfowl, and large birds of prey.
C
Migration
In both arctic and temperate regions, some species of birds are permanent residents, staying in the breeding area all year, although breeding itself is strictly a spring-to-summer phenomenon. Many tropical birds also spend the whole year in the same area; some of these, where seasonality is at a minimum, may nest at almost any time. Most birds of the arctic and temperate regions, and some tropical birds migrate, however—that is, they make regular seasonal movements away from and back to the breeding area. This might be no more than a movement from exposed high mountains down to sheltered valleys for the winter. The opposite extreme is the long-distance migration undertaken annually by many species, the most famous being the Arctic tern, which migrates from the northern latitudes of Eurasia and North America all the way to subantarctic waters.
Long-distance migration raises the intriguing question of how birds find their way. Some fly only by night, others fly over featureless seas; these birds cannot use landmarks as some day migrants seem to do. Scientists now know that no single navigation system exists. Some birds seem to steer by star patterns, and others by the angle of the Sun. At least some birds can detect ultraviolet radiation, the magnetic field of the Earth, and very deep sound vibrations such as those caused by distant ocean waves, but the actual sensory mechanism by which birds translate environmental signals into navigational aids is still a puzzle.
X
BIRDS AND HUMAN BEINGS
Since long before recorded history, as shown by archaeological remains, people have used birds for food. Some tribal peoples still rely heavily on wild birds or their eggs for protein, but with agriculture and civilization came domestication. In most of the world, the birds that are used for human food (mostly chickens, turkeys, guinea fowl, ducks, geese, and pigeons) are bred and raised for that purpose. Eggs are also obtained from domestic birds. Even though hunting for food is no longer necessary in most countries, hunting birds for sport is widespread. Virtually all countries have laws regulating the killing of birds, with most species given protection, and limits are placed on the hunting season and the numbers of game birds that can be taken. Enforcement of such laws, however, is highly variable from country to country.
Birds have played a major role in legends, religious customs, and literature. Feathers have been used for ornamental and ritual purposes by nearly all cultures, and for warm clothing by northern peoples. Birdsongs and calls have inspired tribal religious chants as well as orchestral compositions.
A few species of birds can be detrimental to human interests, principally those that damage fruit and grain crops. Gulls and starlings, attracted to the rubbish dumps that are often near airports, have collided with aircraft, sometimes causing fatal crashes.
By the mid-20th century, the hobby of bird-watching was developing into an industry. People have always enjoyed looking at and listening to birds, but an increasing number have taken an interest in identifying birds and in travelling to new areas to see them. Books and magazines about birds, recordings of birdsong, and binoculars and telescopes for bird-watching are sold in tremendous numbers. Amateurs as well as professionals photograph birds and record their song. Many dozens of travel agencies and individual tour guides specialize in leading bird-watching trips. In common with other forms of tourism, such trips are valuable to the economies of the visited areas and countries as well as to the manufacturers of the bird-watching equipment. Moreover, this enormous popular interest in birds seems to bode well for public support of wildlife conservation measures and for ecological awareness in general. Many amateurs have gone well beyond the “bird-listing” stage and, either on their own or under professional direction, have made important contributions to ornithology (the study of birds).
Scientific classification: Birds make up the class Aves. The earliest known fossil bird is classified as Archaeopteryx lithographica. The fossil bones discovered in Texas were given the name Protoavis. The bird fossils that date back 88 million years include Hesperornis, the giant diver-like diving bird, and Ichthyornis, which might have resembled the living tern in habits.
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