Birds # Pájaros

Aves # Human relationships. Behaviour. Skeleton. Internal organs. Reproduction

  • Enviado por: Andres Kleinman
  • Idioma: inglés
  • País: España España
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BIRDS: THE CLASS AVES GENERAL FEATURES

The smallest living bird is generally acknowledged to be the bee hummingbird of Cuba, which is 6.3 centimetres (2.5 inches) long and weighs less than 3 grams (about 0.1 ounce). The largest living bird is the ostrich, which may stand 2.5 metres (8 feet) tall and weigh 135 kilograms (300 pounds). Some extinct birds were even larger: the largest of the moas of New Zealand and the elephant birds of Madagascar may have reached over 3 metres (10 feet) in height. Among flying birds, the wandering albatross has the greatest wingspan, up to 3.5 metres (11.5 feet), and the trumpeter swan perhaps the greatest weight, 17 kilograms (38 pounds). A Pleistocene condorlike bird, Teratornis incredibilis, had an estimated wingspan of about 5 metres (16.5 feet) and was by far the largest known flying bird. The ability to fly has permitted an almost unlimited radiation of birds, so that they are now found virtually everywhere on earth, from occasional stragglers over the polar ice caps to complex communities in tropical forests. In general the number of species found breeding in a given area is directly proportional to the size of the area and the diversity of habitats available. The total number of species is also related to such factors as the position of the area with respect to migration routes and wintering grounds of species that nest outside the area. In the United States, Texas and California have both the largest number of species recorded (545 and 461, respectively, including both resident and migrant species) and the largest number breeding (300 and 286). Seven hundred and seventy-five species, 650 of them breeding, have been recorded from North America north of Mexico. The figures for Europe exclusive of the Soviet Union are 577 and 420, and the figures for the Soviet Union are 704 and 622. Costa Rica, with an area of only about 51,000 square kilometres (about 20,000 square miles) and a known avifauna of at least 758 species, probably has the most diversified group for its size of any country.

BIRD AND HUMAN RELATIONSHIPS

Wild birds and their eggs have been at least incidental sources of food for humans since their origin and still are in most societies. The eggs of some colonial seabirds, such as gulls, terns, and murres, or guillemots, and the young of some shearwaters (muttonbirds) are even now harvested in large quantities. With the development of agrarian human cultures, several species of birds became domesticated. Of these, chickens, ducks, geese, and pigeons, descended from the red jungle fowl (Gallus gallus), mallard duck (Anas platyrhynchos), greylag goose (Anser anser), and rock dove (Columba livia), respectively, were taken in early and have been selectively bred into many varieties. After the discovery of the New World, the turkey (Meleagris gallopavo), which had already been domesticated by the Indians, and the Muscovy duck (Cairina moschata) were brought to Europe and produced several varieties. Guinea fowl (Numida meleagris) from Africa were also widely exported and kept not only for food but because they are noisy when alarmed, thus warning of the approach of intruders. Besides being a food source, pigeons have long been bred and trained for carrying messages, and the ability of frigate birds to "home" to their nesting colonies has enabled inhabitants of the South Seas to send messages by these birds. For further information on domestication of birds, see FARMING: Livestock farming and Poultry farming. With the development of modern culture, hunting evolved from a foraging activity to a sport, in which the food value of the game became secondary. Large sums are now spent annually on hunting waterfowl, quail, grouse, pheasants, doves, and other game birds. Sets of rules and conventions have been set up for hunting, and in one elaborate form of hunting, falconry, there is not only a large body of specialized information on keeping and training falcons but also a complex terminology, much of it centuries old. Feathers have been used for decoration since early times. Their use in the headdresses of American Indians and various peoples of New Guinea is well known. Feather robes were made by Polynesians and Eskimos; down quilts, mattresses, and pillows are part of traditional European folk culture. Large feathers have often been used in fans, thereby providing an example of an object put to opposite uses—for cooling as well as for conserving heat. Whereas most feathers used in decorating are now saved as by-products of poultry raising or hunting, until early in the 20th century, egrets, grebes, and other birds were widely shot for their plumes alone. Ostrich farms have been established to produce plumes. Large quills were once widely used for writing, and feathers have long been used on arrows and fishing lures. Many birds are kept as pets. Small finches and parrots are especially popular and easy to keep. Of these, the canary (Serinus canaria) and the budgerigar of Australia (Melopsittacus undulatus, sometimes called parakeet) are widely kept and have been bred for a variety of colour types. On large parks and estates, ornamental species like peafowl (Pavo) and various exotic waterfowl and pheasants are often kept. Zoological parks in many cities import birds from many lands and are a source of recreation for millions of people each year. With the rise of agriculture, man's relationship with birds became more complex. In regions where grain and fruit are grown, depredations by birds may be a serious problem. In North America various species of blackbirds (family Icteridae) are serious pests in grainfields; while in Africa a grain-eating finch, the red-billed quelea (Quelea quelea), occurs, like locusts, in plague proportions so numerous that alighting flocks may break the branches of trees. The use of city buildings for roosts by large flocks of starlings and blackbirds is also a problem, as is the nesting of albatrosses on airplane runways on Pacific islands. As a result of these problems, conferences on the control of avian pests are held with increasing frequency. Although birds are subject to a great range of diseases and parasites, few of these are known to be capable of infecting man. Notable exceptions are ornithosis (or psittacosis), caused by one or more viruses that are transmitted directly to man from pigeons, parrots, and a variety of other birds, a serious and sometimes fatal disease resembling virus pneumonia. Encephalitis, an inflammation of the brain, is also serious and is transmitted from birds to man and to his domestic animals by biting arthropods, including mosquitos. Wild birds may also act as reservoirs for diseases that adversely affect domesticated birds. Much work has been done recently on the ecology of viruses, with more and more of them being found in birds. The study of birds has contributed much to both the theoretical and practical aspects of biology. Darwin's studies of the Galápagos finches and other birds during the voyage of the "Beagle" were important in his formulation of the idea of the origin of species through natural selection. Study collections of birds in research museums still provide the bases for important studies of geographic variation, speciation, and zoogeography, because birds are one of the best known of animal groups. Early work on the domestic fowl added to the development of both genetics and embryology. The study of animal behaviour (ethology) has been based to a large extent on studies of birds by Konrad Lorenz, Nikolaas Tinbergen, and their successors. Birds also have been the primary group in the study of migration and orientation and the effect of hormones on behaviour and physiology. Birds feature prominently in mythology and the literature of many countries. Some of their attributes, real or imagined, have led to their symbolic use in art as in language. The aesthetic and recreational pleasures of birdwatching are increasingly being recognized. Man's impact on bird populations has become increasingly strong. Since 1680, approximately 80 species of birds have become extinct, and an even larger number are seriously endangered. While pollution and pesticides are important factors in the decline of certain large species, such as the peregrine falcon, osprey, and brown pelican, the destruction of natural areas and introduction of exotic animals and diseases have probably been the most devastating. Concerted efforts are required to ensure the survival of rare species and to learn as much as possible about them. (R.W.St.)

LOCOMOTION.

Because of their body structure and their feathery body covering, birds are the best fliers among animals, better than the insects and the flying mammals, the bats. There are, however, considerable differences in flying ability among various birds. Penguins cannot fly but spend much of their time in the water swimming with their paddlelike wings; such birds as ostriches and kiwis have rudimentary wings and are permanently afoot. At the other extreme are the long-winged swifts and frigate birds that move from their perches only to fly, never to walk. Most birds alternate some walking or swimming with their flying. Birds usually fly when they have any considerable distance to travel; there are exceptions, however. The mountain quail of California make their annual migrations up and down the mountains by foot. The murres, or guillemots, of the Greenland coast migrate southward by swimming; they begin their journey before the young have grown their flight feathers and before some of the adults at least have regrown their recently molted ones. The Adélie penguins may ride northward on drifting ice floes; at the approach of nesting time they swim back to the Antarctic continent and then walk over the ice to their breeding grounds many miles inland. Flight.Birds fly by flapping their wings, steering mainly with their tails. A goshawk, pursuing its prey through the forest, uses its long tail in making quick turns, and the barn swallow uses its deeply forked tail in making the involved patterns of its graceful flight. Ducks with their short tails have a swift but direct flight. There is, however, such great diversity in birds' tails that the precise size and shape probably is not of critical importance. Comparing a bird to an airplane, a bird's wing is both wing and propeller. The basal part of the wing supplies most of the supporting surface, the wing tip most of the propelling force. A bird's wing has many adjustable features: it can be shortened or lengthened by flexion; the feathers of the tip can be spread or closed; the angle of the whole wing or its parts, on one side or the other or on both sides, can be altered. All these adjustments make the aerodynamics of a bird's wing much more complicated than those of the airplane; consequently, the flight of a bird is much more varied and adaptable. Flying ability varies widely among birds, and different types of wings correlate with different types of flight. Many songbirds use their short, rounded wings mostly to move with quick wing beats from perch to perch or from ground to perch. Ducks have pointed wings that, beaten at high speed, provide rapid flight for long distances. Swallows, terns, and frigate birds have long, pointed wings that enable these birds to fly and manoeuvre gracefully for hours with leisurely wing beats. Large herons with long, broad wings travel far with slow, measured wing beats, while buzzards soar high in the sky on their long, broad wings. Gulls and albatrosses with long, narrow wings sail along the beaches or over the waves with infrequent wing strokes. A hummingbird can whir its tiny wings so rapidly that it can hover as it thrusts its long bill into a blossom; it can even fly backward as it leaves the bloom. The speed with which birds fly varies greatly from species to species, and of course individual birds can vary their speed. The data on the speed of birds' flight are difficult to evaluate. One of the complicating factors is that a bird's speed in relation to the ground may depend on the force of the wind. A bird flying at an airspeed of 40 mph with a 60-mph wind behind it would travel at 100 mph in relation to the ground (1 mile = 1.61 kilometeres). The same bird flying into a 60-mph wind would be losing ground at the rate of 20 mph. Despite the variables involved in determining a bird's speed of flight, the following generalized speeds, based on level flight in calm air, appear to be sound: 10-20 mph—many small songbirds such as sparrows and wrens 20-30 mph—many medium-sized birds such as thrushes and grackles, and larger, long-winged birds such as herons, pelicans, and gulls 20-40 mph—many small- and medium-sized birds such as starlings, chimney swifts, and mourning doves 40-60 mph—the faster-flying birds such as falcons, ducks, geese, and domestic pigeons There are many faster records, often disputed, such as that of 200 mph for an Indian spine-tailed swift in level flight and 170 mph for a golden eagle in a dive. A homing pigeon has been timed at 94.3 mph. The record long-range flight of a bird species in a single season is undoubtedly held by the Arctic terns that migrate from a summering ground in the Arctic to a wintering ground in the Antarctic, travelling more than 11,600 kilometres (7,200 miles) one way. Some long-range flights are made very quickly: a blue-winged teal banded in Canada was recovered 6,100 kilometres (3,800 miles) away in Venezuela only 30 days later; a Manx shearwater, trapped at its nest in Wales and transported 5,200 kilometres (3,200 miles) to Massachusetts and released, returned home in 12 1/2 days. Some very small birds regularly make long water crossings in a single flight. Ruby-throated hummingbirds fly across the more than 500-mile-wide Gulf of Mexico, and many warblers fly from the American coast to Bermuda, a journey of about the same distance.

Flightlessness. Flight, so characteristic of birds, is maintained during the molt in most species by a gradual replacement of the flight feathers. However, ducks and geese, some rails and loons (divers), and auks shed all of their flight feathers at one time, immediately after the nesting season. Not until these feathers are replaced are the birds able to fly again. Most of these are birds that find their food by walking or swimming, as would be expected. Some ducks living in the marshes become very shy and retiring at this season, skulking in the reeds, but geese nesting in the Arctic barrens continue to walk about over the tundra, feeding. In another group of birds, however, the hornbills of Africa and Asia, only the females lose both flight and tail feathers at once; they stay in the nest until the feathers grow out again, being fed during this period by the males. Some birds have completely lost the power of flight during the course of evolution. The close similarity in basic structure of both flightless and flying birds indicates, however, that they all had a common flying ancestor. The rudimentary wings and the flightless condition of the ostrich-like birds and the penguins is a secondary, specialized condition. That flightlessness is a secondary condition is made still more apparent in other flightless birds that belong to families most of whose members are capable of flight. The extinct great auk of the North Atlantic is one of the best known examples of such a flightless bird; the rail family also is noted for having many flightless species living on islands in the Pacific and the South Atlantic. Loss of flight seems to occur most often on isolated islands where there are no mammal predators. In New Zealand, where there are no native land mammals, not only are there many species of extinct flightless moas but also flightless kiwis, penguins, and rails and a duck, an owl, and several songbirds that are nearly flightless. The ostrich-like birds of continental distribution present an apparent contradiction to this correlation of mammal-free island habitats with bird flightlessness. Another adaptation, however, their great size, has enabled these forms to escape the predation of mammals. Walking, hopping, and swimming.Terrestrial birds such as pheasants tend to walk; arboreal songbirds tend to hop as they travel from branch to branch. Parrots often walk along branches, and house sparrows hop when they come to the ground, while palm warblers walk on the ground and some songbirds, such as American robins and European blackbirds, may both walk and hop. Some birds with small feet, such as swifts, hummingbirds, bee eaters, and many hornbills, use their feet only for perching and rarely walk at all. Other birds with robust feet, such as guinea fowl and rails, do most of their moving about on foot. Jaçanas with their greatly elongated toes and nails walk over floating water weeds, and herons with long legs wade in shallow water. The ostrich is probably the fastest running bird; some investigators have credited it with a speed of 50 mph (80.5 kph), at which time the length of its stride was about 25 feet (7.6 metres). The usual position of a bird's body in walking is more or less parallel to the ground. But the penguins, with their feet far to the rear of their bodies, stand upright as they waddle along. When the Adélie penguin, however, makes its trek of many miles over the snow-covered ice to its breeding grounds, it may vary its awkward waddle with periods of tobogganing; i.e., sliding along on its breast and propelling itself with thrusts of its feet. Some water birds have become so adapted to swimming that they are practically helpless on land. In this class are loons, which shuffle awkwardly the few feet from the water to their nests. Swimming in birds is usually correlated with webbed feet, but coots and grebes, with only lobes on their toes, also swim and dive, and gallinules, without either webs or lobes on their toes, commonly swim. On the other hand, frigate birds, with partly webbed feet, never swim. Penguins swim through the water with their wings and use their webbed feet only for steering. Auks use their wings and webbed feet in swimming underwater. Some birds such as the mallard usually swim at the surface, feeding only as far underwater as they can reach by dipping their heads. Other ducks, such as scoters and pochards, commonly dive to the bottom for their food, and cormorants, auks, and loons pursue fish underwater. Sometimes loons are taken at remarkable depths in fishermen's nets and on set lines, indicating that they may dive as deep as 200 feet. Pond ducks, such as mallards and teals, spring straight up from the water's surface into the air in flight, but many swimming birds—for example, coots, grebes, cormorants, and diving ducks—take off with a long spattering run along the surface. (Au.L.R.)

BEHAVIOUR.

Birds depend to a great extent on innate behaviour, responding automatically to specific visual or auditory stimuli. Even much of their feeding and reproductive behaviour is stereotyped. Feather care is vital to keep the wings and tail in condition for flying and the rest of the feathers in place where they can act as insulation. Consequently preening, oiling, shaking, and stretching movements are well developed and regularly used. Some movements, like the simultaneous stretching of one wing, one leg, and half the tail (all on the same side) are widespread if not universal among birds. Stretching both wings upward, either folded or spread, is another common movement, as is a shaking of the whole body beginning at the posterior end. Other movements have evolved in connection with bathing, either in water or in dust. Such comfort movements have frequently become ritualized as components of displays. Many birds maintain a minimum distance between themselves and their neighbours, as can be seen in the spacing of a flock of swallows perched on a wire. In the breeding season most species maintain territories, defended areas ranging from the immediate vicinity of the nest to extensive areas in which a pair not only nests but also forages. The frequency of actual fighting is in birds greatly reduced by ritualized threat and appeasement displays. Birds range from solitary (e.g., many birds of prey) to highly gregarious, like the guanay cormorants of the Peru Current off the west coast of South America, which nest in enormous colonies of hundreds of thousands and feed in large flocks with boobies and pelicans. Auditory signals, like visual ones, are almost universal among birds. The most familiar vocalization of birds is that usually referred to as "song." It is a conspicuous sound (not necessarily musical) that is used, especially early in the breeding season, to attract a mate, to warn off another bird of the same sex, or both. As such it is usually associated with establishing and maintaining territories. Individual variation in songs of many species is well known, and it is believed that some birds can recognize their mates and neighbours by this variation. Many other types of vocalizations are also known. Pairs or flocks may be kept together by series of soft location notes. Alarm notes alert other individuals to the presence of danger; in fact, the American robin (and probably many other species) uses one note when it sees a hawk overhead and another when it sees a predator on the ground. Begging calls are important in stimulating parents to feed their young. Other calls are associated with aggressive situations, courtship, and mating. Nonvocal sounds are not uncommon. Some snipe and hummingbirds have narrow tail feathers that produce loud sounds when the birds are in flight, as do the narrowed outer primaries of the American woodcock. The elaborate courtship displays of grouse include vocalizations as well as stamping of the feet and noises made with the wings. Bill clapping is a common part of courtship in storks, and bill snapping is a common threat of owls. Most birds build nests in which the eggs are laid. Nests vary widely: they may be a scrape in the sand, a deep burrow, a hole in a tree or rock, an open cup, a globular or retort-shaped mass with a side entrance tube, or an elaborately woven hanging structure. The materials with which nests are made also vary widely. Some nests are lined with small stones, others are built of dirt or mud with or without plant material. Sticks, leaves, algae, rootlets, and other plant fibres are used alone or in combination. Some birds seek out animal materials such as feathers, horsehair, or snakeskin. The nest materials may be held together by weaving, sewing, or felting the materials themselves or with mud or spider webs. Swifts use saliva to glue nest materials together and to attach the nest to the supporting structure. In at least one species of swift, the entire nest is made of saliva and is the prized ingredient of birds' nest soup in the Orient. All birds incubate their eggs, except megapodes (mound builders), which depend on the heat generated by decaying vegetation or other external sources, and brood parasites, which lay their eggs in the nests of other species. Murres and the king and emperor penguins build no nest but incubate with the egg resting on top of the feet. In most birds a brood patch is developed. This bare area on the abdomen is edematous (fluid filled) and highly vascularized and is in direct contact with the eggs during incubation. Its development during the breeding season is under hormonal control. When the parent is off the nest, adjacent feathers are directed over the brood patch, and it is usually not apparent. A few birds (e.g., boobies) keep their webbed feet over the eggs during incubation. Incubation takes from 11 to 80 days, depending at least in part on the size of the bird and the degree of development at hatching. Most songbirds and members of some other groups are hatched nearly naked and helpless (altricial) and are brooded until well able to regulate their body temperature. They are fed by the parents until after they are capable of flight. The young of numerous other birds, such as chickens, ducks, and shorebirds, are hatched with a heavy coat of down and are capable of foraging for themselves almost immediately (precocial). Still others, such as the petrels and the auks are downy when hatched but remain in the nest and are fed by their parents. The length of time parents care for young birds varies widely. Young megapodes can fly shortly after hatching and are entirely independent of their parents; young royal albatrosses may spend up to 243 days at the nest and in the area immediately around it before they can fly. The length of time needed to attain independence is related to size and condition at hatching. Ground-nesting birds tend to take less and hole-nesting birds more time than the average. The number of eggs in a set varies from 1 to about 20. Some species invariably lay the same number per clutch (determinate laying), whereas in the majority the number is variable (indeterminate laying). In species of the latter category, clutch size tends to be smaller in tropical regions than in cold ones. There is also a tendency for birds in warm regions to make more nesting attempts in a given season. In the Arctic, where the season is very short, the cycle of breeding and the molt that follows it are telescoped into a minimum of time.

SKELETON.

The avian skeleton (Figure 4) is notable for its strength and lightness, achieved by fusion of elements and by pneumatization (i.e., presence of air cavities). The skull represents an advance over that of reptiles in the relatively larger cranium with fusion of elements, made possible by the fact that birds have a fixed adult size. Birds differ from mammals in being able to move the upper mandible, relative to the cranium. When the mouth is opened, both lower and upper jaws move: the former by a simple, hingelike articulation with the quadrate bone at the base of the jaw, the latter through flexibility provided by a hinge between the frontal and nasal bones. As the lower jaw moves downward, the quadrate rocks forward on its articulation with the cranium, transferring this motion through the bones of the palate and the bony bar below the eye to the maxilla, the main bone of the upper jaw. The number of vertebrae varies from 39 to 63, with remarkable variation (11 to 25) within the cervical (neck) series. The principal type of vertebral articulation is heterocoelous (saddle shaped). The three to 10 (usually five to eight) thoracic (chest) vertebrae each normally bear a pair of complete ribs consisting of a dorsal vertebral rib articulating with the vertebra and with the ventral sternal rib, which in turn articulates with the sternum (breastbone). Each vertebral rib bears a flat, backward-pointing spur, the uncinate process, characteristic of birds. The sternum, ribs, and their articulations form the structural basis for a bellows action, by which air is moved through the lungs. Posterior to the thoracic vertebrae is a series of 10 to 23 fused vertebrae, the synsacrum, to which the pelvic girdle is fused. Posterior to the synsacrum is a series of free caudal (tail) vertebrae and finally the pygostyle, which consists of several fused caudal vertebrae and supports the tail feathers. The sternum consists of a plate lying ventral to the thoracic cavity and a median keel extending ventrally from it. The plate and keel form the major area of attachment for the flight muscles. The bones of the pectoral girdle consist of the furcula (wishbone) and the paired coracoids and scapulas (shoulder blades). The sword-shaped scapula articulates with the coracoid and humerus (the bone of the upper "arm") and lies just dorsal to the rib basket. The coracoid articulates with the anterior (forward) edge of the sternum and with the scapula, humerus, and furcula. The furcula connects the shoulder joints with the anterior edge of the keel of the sternum. It consists of paired clavicles (collarbones) and, probably, the median, unpaired interclavicle. The bones of the forelimb are modified for flight with feathers. Major modifications include restricting the motion of the elbow and wrist joints to one plane, reduction of the number of digits, loss of functional claws, fusion of certain bones of the "hand" (the metacarpals and most of the carpals) into a carpometacarpus, and modification of the elements, especially those toward the tip of the limb (distal), for the attachment of feathers. The wing bones are hollow, and the cavity in the humerus, at least, is connected with the air-sac system. As a general rule, large flying birds have proportionally greater pneumaticity in the skeleton than small ones. The highly pneumatic bones of large flying birds are reinforced with bony struts at points of stress. The humerus, radius, and ulna are well developed. The secondary flight feathers are attached to the ulna, which thus directly transmits force from the flight muscles to these feathers and is therefore relatively heavier than the radius. Two small wrist bones are present: the radiale, or scapholunar, and the ulnare, or cuneiform. The former lies between the distal end of the radius and the proximal part (the part toward the body) of the carpometacarpus. When the elbow joint is flexed (bent), the radius slides forward on the ulna and pushes the radiale against the carpometacarpus, which in turn flexes the wrist. Thus the two joints operate simultaneously. The U-shaped ulnare articulates with the ulna and the carpometacarpus. Anatomists differ on which bones of the reptilian "hand" are represented in the bird's wing. Embryological evidence suggests that the digits are II, III, and IV, but it is possible that they are actually I, II, and III. The carpometacarpus consists of fused carpals (bones of the wrist) and metacarpals (bones of the palm), metacarpals II and III (or III and IV) contributing the greater part of the bone. The phalanges (bones of the "fingers") are reduced to one each on the outer and inner digits and two on the middle one. The primary flight feathers are attached to the carpometacarpus and digits, the number attached to each being characteristic of the various major groups of birds. The pelvic girdle consists of three paired elements, the ilia, ischia, and pubes, which are fused into a single piece with the synsacrum. The ilium is the most dorsal element and the only one extending forward of the acetabulum (the socket of the leg). The ilium is fused with the synsacrum and the ischium, the latter of which is fused with the pubis. All three serve as attachments for leg muscles and contribute to the acetabulum, which forms the articulation for the femur. The leg skeleton consists of the femur (thighbone), tibiotarsus (main bone of the lower leg), fibula, tarsometatarsus (fused bones of the ankle and middle foot), and phalanges (toes). The fibula is largest at its proximal (upper) end, where it forms part of the knee joint and tapers to a point distally, never forming part of the ankle joint. The latter joint is simplified, there being but two bones involved: the tibiotarsus, consisting of the tibia (the so-called shinbone in man) fused with the three proximal tarsals (upper ankle bones), and the tarsometatarsus, resulting from the fusion of metatarsals I through IV and the distal row of tarsals. Metatarsals II through IV contribute most to the tarsometatarsus. The basic number of phalanges (sections) on the toes is two, three, four, and five, respectively; i.e., one more than the number of the toe. Most birds have four toes, the fifth being always absent, but there are many variations in the number of digits, or phalanges, representing reductions of the basic arrangement. The basic avian foot is adapted for perching. The first, or hind, toe (hallux) opposes the other three, and the tendons for the muscles that bend the toes pass behind the ankle joint in such a way that when the ankle is bent the toes are also. The weight of a crouched bird thus keeps the toes clasped around the perch.

INTERNAL ORGANS.

The cardiac (heart) muscles and smooth muscles of the viscera of birds resemble those of reptiles and mammals. The smooth muscles in the skin include a series of minute feather muscles, usually a pair running from a feather follicle to each of the four surrounding follicles. Some of these muscles act to raise the feathers, others to depress them. The striated (striped) muscles that move the limbs are concentrated on the girdles and the proximal parts of the limbs. Two pairs of large muscles move the wings in flight: the pectoralis, which lowers the wing, and the supracoracoideus, which raises it. The latter lies in the angle between the keel and the plate of the sternum and along the coracoid. It achieves a pulley-like action by means of a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula and attaches to the dorsal side of the head of the humerus. The pectoralis lies over the supracoracoideus and attaches directly to the head of the humerus. In most birds the supracoracoideus is much smaller than the pectoralis, weighing as little as one-twentieth as much; in the few groups that use a powered upstroke of the wings (penguins, auks, swifts, hummingbirds, and a few others), the supracoracoideus is relatively large. Avian striated muscles contain a respiratory pigment, myoglobin. There are relatively few myoglobin-containing cells in "white meat," whereas "dark meat" derives its characteristic colour from their presence. The former type of muscle is used in short, rapid bursts of activity, whereas the latter is characteristic of muscles used continuously for long periods and especially in muscles used during diving. The circulatory system of birds is advanced over that of reptiles in several ways: (1) there is a complete separation between the pulmonary (lung) and systemic (body) circulations, as in the mammals; (2) the left systemic arch (aortic artery) is lost, blood passing from the heart to the dorsal aorta via the right arch; (3) the postcaval vein is directly connected with the renal portal that connects the kidneys with the liver; and (4) the portal circulation through the kidneys is greatly reduced. Birds' hearts are large—0.2 to over 2.4 percent of body weight, as opposed to 0.24 to 0.79 percent in most mammals. The avian lung differs from the type found in other land vertebrates, in containing fine tubes (capillaries) through which air passes and through the walls of which gas exchange takes place. Several pairs of nonvascular air sacs are connected with the lungs and extend into the pneumatic parts of the skeleton. The sound-producing organ in birds is the syrinx, located where the trachea (windpipe) divides into the bronchial tubes. The sounds are made by the flow of air setting up vibrations in membranes formed from part of the trachea, bronchi, or both. Muscles between the sternum and trachea or along the trachea and bronchi vary tension on the membranes. The avian digestive system shows adaptations for a high metabolic rate and flight. Enlargements (collectively called the crop) of parts of the esophagus permit the temporary storage of food prior to digestion. The stomach is typically divided into a glandular proventriculus and a muscular gizzard, the latter lying near the centre of gravity of the bird and compensating for the lack of teeth and the generally weak jaw musculature. Otherwise, the digestive system does not vary markedly from the general vertebrate type. Like reptiles, birds possess a cloaca, a chamber that receives digestive and metabolic wastes and reproductive products. A dorsal outpocketing of the cloaca, the bursa of Fabricius, controls antibody-mediated immunity in young birds. The bursa regresses with age, and thus its presence or absence may be used to determine age. The testes of the male bird are internal, like those of reptiles. Intromittent organs are found in only a few groups (waterfowl, cracids, tinamous, ratites). The distal part of the vas deferens (the seminal sac) becomes enlarged and convoluted in the breeding season and takes on both secretory and storage functions. In passerine birds, at least, this enlargement and the adjacent part of the cloaca form a cloacal protuberance, a swelling visible on the outside of the bird. Usually only the left ovary and oviduct are functional. The albumen, membranes, and shell are laid down in the oviduct as the egg moves down it. The gonads and accessory sexual organs of both sexes enlarge and regress seasonally. In the breeding season, the testes of finches may increase over 300-fold in volume over their winter size. Birds are homeothermic (warm-blooded) and maintain a body temperature of approximately 41° C (106° F). This temperature may be 1-1.7° C less during periods of sleep and up to 2° C higher at times of great activity. Feathers, including down, provide effective insulation. In addition, layers of subcutaneous fat add further insulation in penguins and in some other water birds. Reduction of heat loss from the feet in cold weather is accomplished by reducing blood flow to the feet and by a heat-exchange network in the blood vessels of the upper leg, so that the temperature of blood flowing into the unfeathered part of the leg is very low. Birds differ from mammals in lacking sweat glands, hence heat loss is accomplished by rapid panting, which reaches 300 respirations per minute in domestic hens. Some heat dissipation can be accomplished by regulation of blood flow to the feet. In hot climates, overheating is often prevented or reduced by behavioural means—by concentrating activities in the cooler parts of the day and seeking shade during the hot periods. Temporary hypothermia (lowered body temperature) and torpor are known for several species of nightjars, swifts, and hummingbirds. Torpor at night is believed to be widespread among hummingbirds. The heart rate of birds varies widely—from 60 to 70 beats per minute in the ostrich to more than 1,000 in some hummingbirds. The kidneys lie in depressions that are located on the underside of the pelvis. The malpighian bodies, which are the active tubules of the kidney, are very small in comparison to those of mammals, ranging from 90 to 400 per cubic millimetre. More than 60 percent of the nitrogen is excreted as uric acid or its salts. There is some resorption of water from the urine in the cloaca, with uric acid remaining. There is no urinary bladder, the urine being voided with the feces. In marine birds, salt is excreted in a solution from glands lying above the eyes through ducts leading to the nasal cavity.

CLASS AVES Vertebrate (backboned) animals primarily adapted for flight with feathers. Warm-blooded, 4-chambered heart, left systemic arch lost. Lower jaw articulates with cranium via the quadrate; teeth absent in living forms. Reproduction by hard-shelled eggs, nearly always incubated by one or both parents. About 8,600 living species. †Subclass Archaeornithes †Order Archaeopterygiformes (Archaeopteryx).Upper Jurassic; Europe. Teeth set in sockets; long, unfused caudal vertebrae, each bearing a pair of rectrices; keelless sternum; functional claws on digits of hand. Gliding birds, about 50 cm long. Subclass Neornithes †Superorder Odontognathae †Order Hesperornithiformes (Hesperornis, Baptornis).Upper Cretaceous; North and South America. Teeth set in groove in jaws. Flightless, foot-propelled diving birds, 1 to 2 m long. Superorder Neognathae Order Tinamiformes (tinamous).Upper Pliocene to present; Central and South America. Superficially quaillike or grouselike ground-dwelling birds with flat, elongated, and rather weak bills and very small tails. Size 15-50 cm. Order Rheiformes (rheas).Lower Eocene to present; South America. Ostrich-like cursorial birds with very small tails and no aftershaft on the feathers. Sexes alike. Length 90-130 cm. Order Struthioniformes (ostrich).Lower Pliocene to present (the Eocene Eleutherornis may belong here); southwestern Asia and Africa (fossils from southern Europe and southeastern Asia). 2-toed (3rd and 4th) running birds. Males black and white, females brown. Aftershafts, down, and filoplumes absent. Largest living bird; length to 180 cm, height 260 cm, weight 136 kg, egg 1.6 kg. Order Casuariiformes (emus, cassowaries).Pleistocene to present; Australia, New Guinea, adjacent islands. Very large, cursorial (running) birds. Sexes alike, brown (emus) or blackish with brightly coloured wattles and skin on head (cassowaries). Aftershaft very large. Length 130-190 cm. †Order Aepyornithiformes (elephant birds).Pleistocene; Madagascar (upper Eocene and lower Oligocene fossils from Egypt have been placed here). Very large and graviportal (heavy bodied); height to 3 m; egg weight estimated at 10 kg. Order Dinornithiformes (moas, kiwis).Upper Miocene or lower Pliocene to present; New Zealand. Very large (to 3 m tall) and graviportal birds (moas) or smaller (length 30-80 cm); almost wingless, nocturnal, probing birds (kiwis). Order Podicipediformes (grebes).Lower Miocene to present; worldwide. Foot-propelled diving birds with lobed toes, minute tails, and silky plumage. Length 21-66 cm. Order Procellariiformes (albatrosses, shearwaters, fulmars, prions, petrels).Middle Eocene to present; all oceans, but most numerous in Southern Hemisphere. Web-footed marine birds with tubular nostrils; rhamphotheca divided into plates; possess a musky smell. Most have narrow wings and stiff, gliding flight. Length 14-135 cm. Order Sphenisciformes (penguins).Upper Eocene to present; oceans of Southern Hemisphere. Wings flipper-like, for propulsion underwater; webbed feet short and stout; stance upright. Feathers short and dense, molted in patches. Length 40-120 cm (fossil forms to 180 cm). Order Pelecaniformes (pelicans, boobies, tropic birds, cormorants, frigate birds).Paleocene to present; worldwide. Water birds with all 4 toes webbed; bill hooked or straight and sharply pointed. Length 50-180 cm. Order Anseriformes (screamers, waterfowl).Middle Eocene to present. Web-footed birds with broad bills containing fine plates or lamellae (waterfowl); or large-footed marsh birds with chicken-like bills (screamers). Length 29-160 cm. Order Phoenicopteriformes (flamingos).Cretaceous to present; discontinuously distributed in warm regions except Australasia. More varied and widely distributed as fossils. Web-footed birds with long legs, long necks, bent bills with lamellae, and much pink or red in the plumage. Share characters with both Anseriformes and Ciconiiformes, but evidently closer to the latter, with which they are sometimes grouped. Length 91-122 cm (some fossil forms smaller). Order Ciconiiformes (herons, storks, ibises, spoonbills).Upper Cretaceous to present; worldwide except in extreme north. Long-legged wading birds with long bills; feet not webbed. Although usually grouped together, herons and storks may prove to belong to different orders. Length 28-152 cm. Order Falconiformes (diurnal birds of prey).Upper Paleocene to present; worldwide. Diurnal raptors with hooked beaks, long talons, and short (hawks, falcons) or very long (secretary bird) legs or carrion-eating birds with weaker claws and tear-ing bills (vultures, condors). Length 15-150 cm (some fossil forms larger). Order Galliformes (grouse, pheasants, quail, turkeys).Middle Eocene to present; nearly worldwide, except southern South America. Terrestrial or arboreal chicken-like birds; strong, scratching feet; short, rounded wings; feathers with long aftershafts. Length 13-198 cm. Order Gruiformes (cranes, rails, coots, cariamas, bustards). Upper Cretaceous to present; worldwide. Diverse group, ranging from small quail-like hemipodes to large long-legged cranes, marsh-inhabiting rails, swimming coots and fin-foots, and cursorial bustards. The Tertiary phororhacoids belong here, as may the very large Diatryma and its relatives. Length 11-152 cm (fossils to 200 cm tall). †Order Ichthyornithiformes (Ichthyornis, Apatornis).Upper Cretaceous; North America. Superficially gull- or ternlike water birds of uncertain affinities. Length approximately 21 to 26 cm (estimated from reconstruction of fossils). Order Charadriiformes (plovers, sandpipers, gulls, terns, auks).Upper Cretaceous to present; worldwide. 3 basic body plans: Suborder Charadrii—waders (shorebirds), usually feeding on small animals in mud or water, bill variable but often long and used for probing; Lari—web-footed, dense-plumaged water birds feeding by plunging into water for fish, robbing other birds, or scavenging; Alcae—dense-plumaged, web-footed, marine, wing-propelled divers, feeding on fish or invertebrates. Length 13-76 cm. Order Gaviiformes (loons or divers).Upper Paleocene to present; Holarctic. Foot-propelled diving birds with webbed feet and pointed bills. Cnemial crest an extension of the tibia. Length 66-95 cm. Order Columbiformes (sand grouse, pigeons, doves, dodoes). Upper Eocene or lower Oligocene to present; worldwide except in extreme north. Fast-flying birds with pointed wings and weak bills; feed on seeds and fruit. Length 15-84 cm. Order Psittaciformes (parrots, lories, cockatoos).Upper Oligocene to present; throughout tropics, with some temperate-zone species. Often brightly coloured. Strong-flying, seed-, fruit-, or nectar-eating birds with very stout, hooked bills and zygodactyl feet (i.e., outer toe reversed). Length 9.5-99 cm. Order Cuculiformes (turacos, cuckoos, roadrunners).Upper Eocene or lower Oligocene to present; worldwide except in extreme north. Long-tailed birds with zygodactyl or semizygodactyl feet. Feed on both fruits and small animals. Most arboreal, a few terrestrial. Some brood parasites. Length 16-70 cm. Order Strigiformes (owls).Eocene to present; worldwide. Nocturnal raptorial birds with hooked beaks, strong talons, and soft plumage. Length 13-69 cm. Order Caprimulgiformes (nightjars, frogmouths, oilbird).Pliocene to present; worldwide except in extreme north. Concealingly coloured, soft-plumaged, nocturnal birds with weak feet and very large mouths. Most feed on insects caught in flight. Length 19-53 cm. Order Apodiformes (swifts, hummingbirds).Upper Eocene or lower Oligocene to present; worldwide except in extreme north; hummingbirds limited to New World. Rapid-flying birds that feed on the wing on insects and nectar. "Hand" and primaries constitute a relatively great proportion of the wing; feet weak. Length 6.3 to 23 cm. Order Coliiformes (colies or mousebirds).Unknown as fossils. Africa south of the Sahara. Soft-plumaged birds with long, pointed tails and all 4 toes directed forward. Food largely vegetable, some insects. Length 29 to 36 cm. Order Trogoniformes (trogons).Pantropical, except Australasia; upper Eocene or lower Oligocene to present. Extremely soft-plumaged arboreal birds; underparts yellow to red, head and neck often iridescent, tail long, black and white. Feet weak; 1st and 2nd toes directed backward. Food insects and small fruit. Length 23 to 34 cm. Order Coraciiformes (kingfishers and allies).Eocene to present; worldwide except in extreme north. A heterogeneous group of hole-nesting birds. Many with long, pointed bills and blue or green in plumage. All have 2nd and 3rd or 3rd and 4th toes joined at base. Food largely animal, except hornbills, which eat much fruit. Length 9 to 160 cm. Order Piciformes (woodpeckers, barbets, honey guides, toucans).Upper Oligocene (possibly upper Eocene) to present. Zygodactyl (rarely 3-toed) hole-nesting birds. Food insects and fruit. Woodpeckers are modified for climbing. Honey guides are brood parasites. Length 9 to 61 cm. Order Passeriformes (perching birds).Upper Eocene to present; worldwide. The large complex assemblage of perching birds, containing more than half of the known species of birds. Bill, plumage, and habits highly varied. Length 7.5 to 102 cm.

Reproduction.

Courtship behaviour has been described for only a few species of tinamous. Certain species have well-defined breeding periods, and others breed throughout the year. Investigators have reported that courting birds raise the thickly feathered rump and display the brightly coloured undertail coverts. A similar display has been observed in a frightened Crypturellus: the bird presses the breast to the ground, raises the rump, spreads the terminal feathers like a fan, and exhibits the sharply marked undercoverts. Courting birds also have been observed to chase each other around on the ground. Multiple mating is the rule among tinamous, although a few species such as the ornate tinamou (Nothoprocta ornata) maintain stable pairs. All forms of polygamy exist, the conditions varying between and even within species. Many species have uneven sex ratios; preponderance of males seems to be more frequent, the ratio reaching four to one in the variegated tinamou (Crypturellus variegatus). The ratio in the ornate tinamou is about one to one. The nest, a shallow depression in the ground, is constructed and defended by the male. The eggs are among the most beautiful of all bird eggs, always monochromatic and highly glazed. The colours include light chocolate brown, near black, purple, dark bluish green, light yellowish green, and gray when laid, but the shell pigments fade when exposed to light. One hen, or more, places her eggs in the nest of a male; when several females provide the eggs, the clutch may become quite large, eight to 16 eggs. Incubation, which lasts 17 to 21 days, is done entirely by the male, who broods and guides the chicks for several weeks after hatching. The chicks, blotched and streaked like the young of the rheas, are able to run as soon as they are hatched. When frightened, they squat and freeze, becoming almost invisible. PALEONTOLOGY AND CLASSIFICATION

There is no doubt that the tinamous represent one of the oldest stocks of birds on the South American continent. To date, three genera of fossil tinamous, of one species each, have been described from a single deposit from the Miocene of Argentina (about 10,000,000 years ago). The majority of other fossil tinamous, mostly representing species still extant, has been discovered at scattered sites from the upper Pleistocene (less than 1,000,000 years ago) of South America. Many authors have noted anatomical and biological resemblances between the tinamous and rheas, or nandus (Rheidae). The structure of the bony palate, an important feature in the taxonomy of ratite birds, quite clearly links the two groups, but most authorities prefer to maintain them as separate orders, Rheiformes and Tinamiformes, each with a single family, respectively, Rheidae and Tinamidae.

Classification.The following classification of the order is universally accepted.

ORDER CASUARIIFORMES Large graviportal (ponderous) flightless birds with 3 toes. Other physical features described in Form and Function, above. Monogamous. Eggs green in colour. Family Casuariidae (cassowaries) Beak laterally compressed. Prominent horny casque on mesethmoid, nasal, and frontal bones of the skull. Coracoid bone of the shoulder much shorter than in emu. Bony union (symphysis) between various posterior elements of the pelvis may be present or absent. Femur not pneumatized by air sac system. Cervical vertebrae 18 or 19. Wing with 5 quills modified as long hollow spines. Pendulous coloured wattles on throat and neck (except in dwarf cassowary). Three species. Family Dromaiidae (alternatively Dromiceiidae; emu) Beak flattened dorsoventrally. Head not casqued. Pubic symphysis lacking. Femur pneumatic. Trachea with an aperture in front, leading to an inflatable neck sac. Pendulous wattles absent. Fossil record limited to the Pleistocene of Australia and nearby islands. One Recent species. Family Dromornithidae Fossil only; Pleistocene of Australia; 2 genera, 2 species. (D.L.Se.) Podicipediformes (grebes)The order Podicipediformes includes only the grebes, foot-propelled diving birds best known for the striking courtship displays of some species and for the silky plumage of the underparts, which formerly was much used in millinery. The speed with which grebes can submerge has earned them such names as water-witch and helldiver, while the position of the feet near the tail is responsible for the early English name arsefoot, from which the family name, Podicipedidae, was derived. Adult grebes range in weight from less than 150 grams (five ounces) to more than 1.4 kilograms (approximately three pounds) and in total length from 21 to 73 centimetres (eight to 29 inches). They vary principally in bill shape and ornamentation of the head. The group is found on all of the continents and on many island groups as well. However, it is best represented in temperate regions. Seven species each are found in North and South America, five in Eurasia, and three each in Africa and Australia. The species range from conspicuous and gregarious to solitary and skulking.

Importance to man

.The tube-nosed birds have been of considerable local economic importance as a source of protein food, feathers, and oil wherever humans have colonized or have been able to raid the coastal and oceanic islands where they breed. This situation has resulted in the partial or complete extermination locally of certain species. Humans, moreover, have been responsible for the introduction of various predators, including rats, pigs, and cats. In regions where bird populations have survived, people have continued to harvest the eggs, the plump young birds (at fledging time), or both. Many thousands of slender-billed, or short-tailed, shearwaters (Puffinus tenuirostris) are taken on the Bass Strait islands off Tasmania and sold fresh, salted, or deep-frozen as "muttonbirds." In all likelihood, the name muttonbird was derived from the use of the flesh as a supplement for mutton by the early settlers of New South Wales. The numbers of muttonbirds now harvested are regulated so as to preserve a substantial breeding stock. In New Zealand the Maori people have harvested young titi (shearwaters of several species) from time immemorial, a right assured them in perpetuity by treaty with Queen Victoria. On the other side of the world, hundreds of Manx shearwaters (Puffinus puffinus) were formerly collected for food and as lobster bait on the Welsh islands of Skomer and Skokholm, which are now nature preserves estimated to contain about 200,000 Manx shearwaters and 2,000 storm petrels (Hydrobates pelagicus). On the Tristan da Cunha Islands in the South Atlantic, resident islanders harvest the eggs and squab (young) of a large, mixed seabird population, which includes more than 6,000,000 greater shearwaters (Puffinus gravis). The harvesting of northern fulmar petrels (Fulmarus glacialis) is an ancient practice among peoples inhabiting the cool northern coasts where the birds choose to breed. In Iceland about 50,000 fulmars were taken annually between 1897 and 1925; however, the occurrence in 1939 of psittacosis (a virulent avian disease) among processors of the birds resulted in prohibition of the use of fulmars for food. During the early 17th-century colonization of Bermuda, millions of cahows, or Bermuda petrels (Pterodroma cahow), were exterminated by the colonists. For nearly 300 years the species was believed extinct, but in 1951 a few pairs were discovered nesting on an offshore islet, where a remnant now survives under strict protection. The related black-capped petrel, or diablotin (P. hasitata), of the West Indies was also thought extinct (because of predation by humans, rats, and mongooses) until in 1961 a substantial population, estimated to number at least 4,000 birds, was found breeding in the inaccessible forested crags of Hispaniola. In the 18th and 19th centuries, huge numbers of albatrosses were taken for food (largely by whalers) and for the millinery trade. With the disappearance of sailing vessels, changes in fashions, and the establishment of many nesting grounds as sanctuaries, such predation has virtually disappeared, but albatrosses have not entirely escaped stress at the hands of man. On Sand Island in the Midway Atoll, Laysan albatrosses (Diomedea immutabilis) increased from a few pairs in 1900 to about 60,000 pairs in the early 1960s, the increase resulting from shelter provided by introduced vegetation. The use of the island by aircraft after 1935, air raids by the Japanese during World War II, and the loss of 30,000 birds humanely killed by the U.S. Navy (in a control program designed to reduce collisions between birds and aircraft) did not deter the albatrosses from this favoured nesting area. The control program was abandoned after the discovery that levelling certain sand dunes effected a 70 percent reduction in collisions by removing the updrafts near aircraft runways.

Distribution.

The majority of procellariiforms breed in the Southern Hemisphere, but several species migrate thousands of miles north across the Equator to winter in the northern summer seas, where they molt, feed, and rest in preparation for the return home in the southern spring. Similarly, species that breed in the Northern Hemisphere also live in perpetual summer by migrating far south to winter in the southern summer. A number are less migratory and do not cross the Equator. Several species are almost sedentary, chiefly smaller petrels breeding in tropical and subtropical latitudes, and the subantarctic prions (Pachyptila) and diving petrels. All latitudes of the unfrozen oceans are thus occupied, but there are fewer living in the calm equatorial region, where there is little wind to lift their long wings and where the pelagic (open-ocean) crustacean food on which so many seabirds basically depend is scarce. The zone of upwelling water in the windy latitudes of the Antarctic convergence, between 40° and 60° south latitude, is richest in the shrimplike krill (Euphausia species), attracting surface-feeding tubinares and the diving penguins, prions, and diving petrels. Some feed along the edge of the ice of the Antarctic continent, and four tubinares actually breed on its shores: the Antarctic fulmar, the giant petrel, the snowy petrel, and the tiny but very numerous Wilson's petrel; the nesting burrows of the last may be blocked by snow for days during the protracted breeding season. The only tubinare nesting near the ice limits in the high Arctic is the fulmar, which reaches Franz Josef Land, Greenland, and the Arctic Circle north of the Aleutian Islands. Of the albatrosses (family Diomedeidae), only the two Midway species and the short-tailed albatross (Diomedea albatrus) nest well north of the equatorial doldrums. The latter was brought close to extinction by plume hunters and by a volcanic eruption at its nesting island of Torishima. There were enough immature birds at sea at the time to allow a partial recovery, and some 60 individuals were counted there in 1969. Nine albatross species range the Southern Hemisphere, gliding on the eternal winds of the "Roaring Forties" (the region between 40° and 50° latitude) and moving north with the food-rich cold currents along the west coasts of South America, South Africa, Australia, and New Zealand. One species, the waved albatross (Diomedea irrorata), is unique in that it breeds only in the Galápagos Archipelago at the Equator, where probably not more than 3,000 pairs nest on Hood Island. The family Procellariidae includes the larger petrels, such as the northern and southern fulmars (Fulmarus glacialis and F. glacialoides), the gadfly petrels (Pterodroma), several genera of shearwaters, and the prions or whalebirds. Several of the shearwaters and larger petrels breed in burrows far inland on mountain crags in the Andes, West Indies, Madeira, and New Zealand. The largest member of this family is the giant, or stinker, petrel (Macronectes giganteus), an albatross-like scavenger and circumpolar wanderer with a heavy beak and wing span of eight feet. Smallest are the prions, four species of small, stocky, little-studied birds, 22 to 30 centimetres (nine to 12 inches) long, with broad bills and a restricted cold-water range; they breed on sub-Antarctic islands, keeping much to the water, as do the diving petrels. The storm-petrel family, Hydrobatidae, ranges both hemispheres but is strongest numerically in the Pacific, where Halocyptena microsoma, the least petrel of Baja California, rivals the Atlantic storm petrel as the smallest procellariiform. The word petrel ("little Peter") derives from a habit of the storm petrels of walking on the waves. The diving petrels form a family (Pelecanoididae) and genus (Pelecanoides) with four species. They are small, rather sedentary, coast-dwelling birds confined to cool southern islands, including Tristan da Cunha, the Falklands, New Zealand, and southeastern Australia. The common diving petrel (P. urinatrix) is circumpolar; the largest species, the Peruvian (P. garnotii), has followed the Humboldt current along the west coast of South America and breeds from Chile to Peru; the Magellanic (P. magellani) is confined to the tip of South America; the Georgian (P. georgicus) breeds on South Georgia, Macquarie, and Auckland islands. Diving petrels are specialized birds with short, black-and-white bodies and closely resemble externally and in habits the small auks (family Alcidae, order Charadriiformes) of the Northern Hemisphere.