Natural history ( bird )

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. Compared to the parts of 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 kilometres). 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. For further information on bird migration, see migration.

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 (see photograph) 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.

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 (see photograph). 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.

Feeding habits

The earliest birds were probably insectivorous, as are many modern ones, and the latter have evolved many specializations for catching insects: swifts, swallows, and nightjars have wide gapes for catching insects on the wing; some woodpeckers can reach wood-boring grubs while others can catch ants by probing anthills with their long, sticky tongues; thrashers dig in the ground with their bills; tree creepers and woodhewers probe bark crevices; and warblers glean insects from many kinds of vegetation. Raptorial birds have evolved talons and hooked bills for feeding on larger animals, and vultures have bare heads and tearing bills for feeding on carrion. Herons have spearlike bills and trigger mechanisms in the neck for catching fish, while kingfishers, terns, and boobies plunge into the water after similar prey. Long-billed waders probe for worms and other invertebrates. Of the many kinds of birds that feed on plant material, most use seeds, fruit, or nectar, which are high in food value; leaves and buds are eaten by fewer species. While some kinds of birds feed entirely on a single kind of food, others may take a wide range of foods, and many have seasonal changes.

[tags]natural, bird history, birds, species, fly[/tags]

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