- Look at the way the Japanese have painted birds. . . . The system is simple. They have sat down in the countryside and have for a long time watched the birds in their flight. By dint of watching them they have ended by understanding their movements.
- Auguste Renoir
The wing of a bird in flight has no fixed shape. Its shape is constantly changing in response to the constantly changing pressures of the air through which it moves, or to suit changes in course or speed. Man has never invented anything at once so strong and so delicate, anything so sensitive in its adjustment to the continuously varying conditions it has to meet.
A bird's foot, being a relatively simple mechanism, changes shape only to the extent that it folds or unfolds at set joints; but the changes of shape that a bird's wing undergoes are fluid throughout. It shortens or lengthens, narrows or broadens, and the curvature of its surfaces is constantly being modified, subtly or drastically, slowly or suddenly. No one shape is quite retained for more than a moment. This is true even of the great albatross that banks and wheels, rising into the sky and coasting down to skim the running waves on what we think of as set wings. A myriad small adjustments, at least, are constantly occurring.
Because the wing of a bird in flight is motion itself, no photograph or drawing can do it full justice. The representation can never be more than a half truth. When, for example, I watch the flying seabirds at close range on their nesting grounds in the Shetland Islands, far to the north of the Scottish mainland, it is clear that the wing of the fulmar is much narrower than that of any gull. But I have photographs of flying fulmars in which the wing appears as broad as a gull's simply because of the way it is being held at the moment.
Scientists may speculate whether, in the course of evolution, form or function has come first. However they have developed in relation to each other, they are inseparable, each giving meaning to the other. Nothing illustrates this more beautifully than the flight of seabirds.
Birds have various modes of moving through the air, with all gradations between them. In one mode commonly associated with fast and direct flight, the wings move rapidly up and down like two sticks oscillating through a short arc. This is typified by the members of the auk family—the razorbill, the murres, the black guillemot, the puffins—and by the fulmar, which alternates a succession of rapid beats with intervals of gliding,
In another mode, the bird progresses by successive impulses that represent distinct wing-strokes separated by pauses. For this pulsating mode, which is the normal mode of such typical terns as the common and the Arctic, the wings are jackknifed. This is to say that they reach out and forward from the body to the midway joint, from which they angle sharply back—like a half-folded jackknife. The frigate or man-of-war birds of the tropics, although so much larger than the terns, also fly this way. So, for the most part, do shearwaters.
The pulsating mode, as it is less mechanical, is more flexible than the oscillating mode. The fulmar tends to move straight and fast, whereas the tern may alter course and speed with each stroke of its wings. It can stop and go, dodge and dart, pausing to see whether a glimmer in the wave below is a fish to dive at—and, if it is not, resume its full speed at one stroke.(However, when flying long distances in calm air, as when migrating, the terns have a shallower and steadier wing-beat. At other times, when simply holding their positions in the face of the wind, they wave their wings slowly and gently.)
Whether a wing is jackknifed or straight does not depend on the species. A fulmar can jack knife its wings like a tern in order to tip down into the updraft along the face of a cliff; a tern can extend its wings like a fulmar to brake its flight.
The jackknifed wing, however, is the only one for sprinting. This is exemplified among land birds by the peregrine falcon, among seabirds by the parasitic jaeger. The jaeger, living largely by the practice of piracy, combines power and elegance in its flight as does no other bird. One circles on extended wings off the high bluffs and cliffs of Shetland where puffins are nesting. When it sees a puffin approaching from the sea with a sand eel held crosswise in its bill, it sets off suddenly with jackknifed wings striking down in stroke after stroke, seeming to reach its top speed with the first stroke. The puffin may or may not have time to save its provender by plunging vertically to the water and disappearing beneath the surface. If not, it can save itself from being disabled only by dropping the fish for the jaeger to pick out of the air in one long and lovely swoop.
The reason why the wing had best be jackknifed for sprinting is clear. The outer half, because it lags after the inner on the down stroke, slopes upward from joint to tip, its long terminal feathers being further bent upward by the pressure of the air beneath them. To the extent that this upward sloping segment points backward rather than merely outward, it propels the bird forward on each downbeat. Moreover, the air is less resitant to the jackknifed wing.
In seabirds of any size the oscillating mode of flight tends to go with relatively narrow wings, which may be beaten the more rapidly because, by their narrowness, they present less surface to the resistance of the air. They cut through it as broad sails could not. The limited wing-surface, in turn, may represent an adaptation to an environment of strong winds. For, the stronger the wind that supports it, the less wing-surface the bird needs, while more surface than was needed would make its progress against the wind that much slower and more difficult, as well as requiring more muscle.
Because the wings of gulls are broader than those of fulmars, their wingbeats are relatively sluggish. While they cannot, therefore, match the fulmar's speed in straightaway flight, or the sprinting ability of the jaeger with its jackknifed wing, they are better suited to drifting and soaring in light winds. The wings of the fulmar are adapted to the great winds of the ocean far from land, those of gulls to onshore and offshore breezes.
Even on a windless day a gull can rise lightly from sea or land in one spring; and it can feed from the surface of the water in hovering flight. A fulmar cannot properly hover (although it can hang in a sufficiently powerful updraft against a cliff), and it can take off from the water only by a taxiing run in which it has to work its feet strenuously. This means that, where gulls and fulmars are feeding together on offal dumped from fishing boats, the gulls get there first.
On windless days in Shetland the fulmars launch themselves with difficulty from the stone walls on which they sometimes roost. Unable to rise until they have gained sufficient air speed, they lose altitude until they are bumping along the ground, running with their feet and beating their wings strenuously to achieve the speed at which they become, at last, completely airborne.
It is surely no accident that the kittiwakes, which are the only gulls that range the open ocean rather than the shores, have wings distinctly narrower than those of the typical gulls. The difference in the breadth of their wings and the consequent mode of their flight is well seen during the nesting season in Shetland, when their presence in the same sky with typical gulls makes direct comparison possible.
The kittiwakes do not, however, have the fulmar's mode of flight. They are, rather, intermediate between the typical gulls and the terns, although closer to the former. They hold their wings partially jackknifed as a rule, the inner halves extending forward more than in the typical gulls, although the outer halves do not sweep back as sharply as in the terns. They have a more pulsating flight; and when, like the typical gulls, they circle on fixed wings outstretched, they do so only momentarily.
The kittiwakes are like the terns, too, in their diving ability. I have seen a flock of them over a school of fish diving vertically from on high with wings folded, all disappearing beneath the surface for an appreciable time.
In flight they differ from the typical gulls, and from the terns too, (although not from fulmars), in that head and body appear to be all one, as if the birds were flying torpedoes, whereas the typical gulls and the terns show the constriction of a distinct neck separating head from body.
The pulsating flight of the kittiwake combines irregularity with regularity in the way that distinguishes everything in nature from anything that is merely mechanical. The appreciation of this as a fundamental characteristic of nature was well expressed by the French painter Renoir in his note-book—as quoted in the biography written by his son. Noting that the great artists of the best periods imitated nature in that "they created irregularity within regularity," he wrote:
The Earth is not round. An orange is not round. None of its segments has either the same form or the same weight. Open them and they will not have the same number of seeds, and the seeds will not be alike. . . . I do not want a column to be any rounder than a tree.
I never saw a kittiwake fly in a perfectly straight line, even when purposefully maintaining a single direction. Instead, there is a slight wavering or yawing that goes with a habit of tilting from side to side. The wingbeats have a regular rhythm within which they are irregular. The tilting and wavering is accompanied by an alternate quickening and retardation of the beat, but all within the limits of the measure.
In his Principles of Natural Knowledge, Alfred North Whitehead wrote:
A rhythm involves a pattern, and to that extent is always self-identical. But no rhythm can be a mere pattern; for the rhythmic quality depends equally upon the differences involved in each exhibition of the pattern. The essence of rhythm is the fusion of sameness and novelty; so that the whole never loses the essential unity of the pattern, while the parts exhibit the contrast arising from the novelty of their detail. A mere recurrence kills rhythm as surely as does a mere confusion of detail.
Anyone who studies music or poetry appreciates the need of such irregularity within regularity, but how few have traced it back to nature as Renoir did! Verse with only a mechanical beat is doggerel rather than poetry, and no one sensitive to the quality of music could long endure listening to a composition that had no more rhythmic variation than a metronome—even though the variation occurred, as it must, within a fixed measure.
Usually, when we think in these terms, we talk about nature as the great artist; but nature provides the original that serves as model to the human artist. It came first. While we may say that the flight of the kittiwake is like music, with rests, grace-notes, and turns that have their place within a larger rhythmic pattern, it would better accord with the logic of being to say of a piece of music that it followed the model provided by the flight of the kittiwake.
Of two wings with the same surface area, one may be long and narrow, the other short and broad. Species that inhabit woodland, like our common crow, need the shortest possible wings to thread their way through the mazes of branch and twig; so that their wings, in order to have enough surface, must make up in breadth for what they lack in length.
I know of no reason why breadth of wing, in itself, should make possible a steeper angle of climb. It is a fact, however, that crows, pheasants, American black vultures, herons—all such broad-winged birds can climb the air at remarkably steep angles. Perhaps the explanation is that the lifting power of a wing is greater in the degree to which it is cambered (arched from front to back), and a broad wing lends itself to cambering as a narrow one does not. A heron, its wings broad and cambered alike, is a slow if buoyant flier. In the days of falconry, when a peregrine was loosed against a heron the heron would try to remain above its pursuer by climbing steeply if slowly, while the falcon, with its flat rate of climb, would try to get above it by the speed of its flight in wide circles that sometimes took it beyond the horizon.(The narrowness of wing in the fulmar, the razorbill, the murres, and the puffins explains their difficulty in rising from the surface of the water at all in the absence of a powerful wind. A mallard, with its broad wing, has no difficulty.)
Two European birds, the lapwing and the wall creeper, have wings that seem broader in their outer than in their inner halves. In the case of the lapwing this is associated, presumably, with its ability to take off from the ground at the steepest angle; in that of the wall creeper with its habit of flying almost vertically up the face of a cliff, albeit in stages with brief touch-downs between.
The lapwing is the only bird I know with broad and rounded wings that holds them in the jackknifed position for its typical flight. In that flight it swerves constantly, from side to side or down and up, as if governed by a succession of erratic and ecstatic impulses.
(Despite my citation of the little wall creeper, my observations apply, in general, only to birds above a certain size; for the relation of mass to volume is different, creating different circumstances, in small birds like swallows and sparrows.)
Because the biggest seabirds, such as the great albatrosses, have little or no need either to fly slowly or to climb steeply, they have the longest and narrowest wings of all. For narrow wings are more efficient than broad ones in that the turbulence of the air caused by the passage of their tips is less. The wings of the wandering albatross are like strips of whalebone that extend up to 11½ feet from the tip of one to the tip of the other. A great albatross may spend most of its life sailing with outspread wings over the unbroken ocean of high southern latitudes, circumnavigating the earth repeatedly without coming to land. Under such circumstances it never has occasion to climb steeply or to reduce its speed drastically. However, when it does have to come to land for its nesting, every other year during its adult life, its inability to reduce its speed sufficiently may cause it to tumble tail-over-bill upon touching down. To become airborne again, it generally has to walk to the top of some hill where it can take off into the wind.
In my experience, when a great albatross remains in attendance on a ship that has a following wind, it has to circle the ship continuously, presumably because it cannot reduce its speed enough to hang in her wake like a gull.
Many large land birds with broad wings, such as the turkey vulture, glide and soar on wings that slope upward in a flat V. It may be that this enables them to soar in tighter circles upon thermals, those columns of air that rise above ground heated by the sun. Seabirds, however, have no such need to limit their soaring to tight circles. All those that are large enough to glide or soar hold their wings more or less bowed, the tips below the horizontal. This bowed position is often extreme in the great albatrosses; so that, seeing them in their wheeling flight, one could believe that their springy whalebone wings had invisible weights hanging from the tips. The reason, I surmise, is that in the bowed position the body is supported chiefly by the inner part of the wing, which is held against the air with less muscular strain. It is somewhat as if the outer segments should be partially folded away when they were not needed. Bowing the wings shortens sail.
Many seabirds have an excess of wing-surface for ordinary circumstances. This is demonstrated on the nesting grounds of parasitic jaegers, kittiwakes, and Arctic terns, the young of which may rise in flight, albeit with some effort, when wings and tail are still only half grown.
Even when they are not sprinting, jaegers and terns may hold their wings in the jackknife position, half folded, simply because they have too much sail to carry it fully spread to the wind. They reef it so as not to be blown away.
The wings of birds are so infinitely varied in form and use that one finds exceptions to almost any generalization one may make about them. Thus the great skua has relatively broad although pointed wings, at least as broad as those of the slow-flying herring gull, yet it can sprint like its relative the parasitic jaeger—and does so for the same piratical purpose. Here the explanation is in the development of the pectoral muscles by which it works its wings, as shown by its immense girth. A great skua sprinting after its prey represents the ultimate in power flight.
Even the most extreme specialization comes up against limits set by circumstances. Perhaps the wings of the wandering albatross would be shorter than they are, and therefore not have to be shortened by bowing in ordinary flight, if they were not needed, however rare the occasion, to brake the great bird's momentum when it comes in for a landing on its nesting grounds.
Every bird, even the albatross, must sometimes come to rest, on water or on land, with wings folded away. In fact, if the choice is between being airborne all the time or never, it must be never. Ostriches and kiwis lost the power of flight completely at some stage of their evolution, so that they could be more efficient on land. The penguins lost it so that they could be more efficient under the water.
The penguins once had dual-purpose wings for aerial and underwater flight alike. To the extent that their wings were adapted to aerial flight they were inefficient for underwater flying, and vice-versa. Because underwater flying, by which they caught the marine organisms on, which they lived, was more important in the particular environmental circumstances of their lives, they at last specialized in the former at the expense of the latter; so that today their wings are mere flippers, excellent for propulsion under the water but useless for rising above it.
One may doubt that circumstances in the northern hemisphere, to which they are confined, will allow the surviving members of the auk family to lose their powers of aerial flight. (The circumstances of the penguins, confined to the southern hemisphere and breeding where there were no mammalian predators, have been quite different.) The great auk did lose its powers of aerial flight, like the penguins, but the result was its extermination when man at last found his way to its nesting grounds and proceeded to harvest its numbers. The last one died in 1844.(I daresay whoever killed it then proceeded to wash his hands.)
The wings of all the other auk species—of razorbills, murres, guillemots, puffins, dovekies—are compromises between adaptation for aerial flight and adaptation for underwater flight. Requiring a smaller surface for the denser medium but a larger for the less dense, they undoubtedly have too much for the one and too little for the other.
Looking down from the top of the cliffs on which they breed, one may sometimes see the common murres in their underwater flight, which presumably typifies that of all the auks. The wings are outstretched only to the wrist-joint, the outer segments that bear the primary flight feathers being folded so as to form spikes that point backward—thus reducing the entire wing surface for the denser medium but adding to the forward propulsion of each wing-stroke. The bird proceeds by separate successive strokes, each shooting it forward and being followed by a pause during which it loses momentum.
If the fully spread wings are too large for use under water, they are smaller, relative to the stout bodies, than they ideally should be for use in thin air; which accounts for the rapidity with which the auks generally have to beat them in overwater flight. A black guillemot taking off from the surface of a tossing sea sets its wings a-humming—but it may still collide with successive wavecrests before it gets free, if it does at all. (When a guillemot, murre, or puffin has to get out of the path of a ship quickly, it generally does better by diving, but it is often moved to try the alternative nevertheless.)
On the other hand, skimpy wings may be excellent in a high wind, especially if it takes the form of a powerful updraft against the sea-cliffs where puffins, murres, and razorbills nest. The wings of these three species are excessively narrow; but in a roaring updraft against the cliffs they may suffice to hold the bird suspended without themselves moving, except for an occasional tremble.
One may say of the typical gulls (as distinct from the kittiwakes) that they are not true seabirds because, roosting every night on land, they never range far out from shore. The auks and the fulmar, however, when they are not at their nesting grounds, live night and day on the open ocean, sleeping on the billows, drinking only salt water. If it were not that they had to come to land for nesting, their wings might become more specialized, adapted to the pelagic environment only. As in the case of the albatross, however, their wings must retain at least enough braking ability to enable them to land on hard ground without crashing.
Fulmars, for all that they are birds of the open ocean, are highly specialized for patroling flight along the cliff-faces. There is a mystery here in that, although they spend a large part of their lives in such flight, one can see no useful purpose that it serves. It seems a pure pastime.
The patrol requires spectacular dexterity in flight. Recently, standing at the top of a cliff where it was broken inward to form a trap for the howling wind from the sea. I spent the best part of a quarter hour watching a fulmar ten or a dozen feet from my face. It was riding the turbulence of the updraft like a cowboy riding a bucking bronco. It kept its position, simply allowing itself to be carried backward or forward, to one side or another, within strict limits of distance—like a kite at the end of its string. Wings and tail were constantly responding by twisting and bending, folding and unfolding, to the sudden buffets from below.
Another fulmar in the same wind would slip slowly, inch by inch, down the broken, sloping face of what was half cliff, half bluff, almost in contact with the surface of which it followed every irregularity. When it had got low enough, it would allow itself to be lifted quickly to the top to resume the slow, slipping descent.
One of the fulmar's special adaptations for such cliff-hanging is a capacity to make its wings tremble in order to give some slight extra lift or propulsion. The wholly unrelated puffin, which also patrols the cliffs and steep bluffs (although generally farther out), has the same adaptation. It may be able to hold its position in the most powerful updrafts without actually beating its wings; but it is so much smaller than the fulmar, with so much less mass to oppose to the wind, that it must frequently vibrate them to avoid being blown backward.
The puffin may also vibrate its wings when it coasts at a steep angle from its high nesting-sites to the water. The larger razorbilled auk does the same. On the other hand, razorbills also descend to the water by simply waving their wings slowly like big butterflies.
The puffins commonly nest in burrows on steeply sloping bluffs that face the sea. Unlike the birds that nest on ledges of vertical cliffs, they generally come in to the landing platforms in front of their burrows from above, and their wings are barely adequate to such an approach. They beat them hard as they draw close, but at the last instant seem to give up and drop vertically, hitting the ground with a thump.
If there were talking puffins with philosophical minds (and, more than any other birds I know, they look as if they had philosophical minds and could talk), I can imagine one saying: "We have to use our wings to catch fish under water; we have to use them to take off from the water and fly over it; we have to use them, in accordance with the rules of our nature, to sail in the updraft against cliffs and bluffs; and we have to use them to come in to our landings at our nesting burrows, Because our wings could not possibly be perfectly adapted to all these purposes at once, they necessarily represent a compromise. We are not, after all, like the penguins, who have to do only one thing with their wings—and can do only one thing with them."
Although there is less mystery about the flight of birds than there used to be, the wonder continues everlasting. Aerodynamic theory, as developed in connection with the design of aircraft, together with slow-motion pictures of birds in flight, has taught us much about how the wings of birds lift and propel them.(To the mind of one layman, at least, what remains truly mysterious is the flight of a butterfly opposing its weight to the wind.)
The everlasting wonder is partly in the structure of the wing itself, at once so light and so strong, so complex in the multitude and diversity of its parts, so flexible in their mutual connections and articulations, so harmonious in their combination to form a single whole. The wonder is also in the diversity of its adaptations to the diverse requirements of so many species, from the storm petrel, hardly bigger than a butterfly, to the great albatross.
What is no less a wonder is the control exercised by the bird, from some center in its head, over all the shifting surfaces at once. The constantly changing shape of a bird's wing in flight, as of its tail, is produced by a complex and subtle interaction between the varying pressures of the air and the impulses that come from inside the bird itself in the form of what is equivalent to countless decisions per instant.
The sensitivity of all these infinitely rapid adjustments may be seen in the flight of a wandering albatross over the waves of the South Pacific. Banked on set wings, the bird moves perpetually in a great wheel tilted toward the surface, so that it rises high only to come sweeping down again until the lower wingtip is skimming the waves, almost touching them but never quite. The point of the wing follows the moving contour of a sloping and rippled wave-surface so closely that it actually describes the successive ripples, rising and descending over them in a fluttering, shimmering movement. Although moving at high speed and remaining virtually in contact with the rough and changing contour of a running wave, the wingtip never touches it, never scores the surface, never gets hooked in it. The bird's eye, which in any case must be occupied elsewhere, is some five feet away from the wingtip. Surely the delicate control involved in this constantly repeated performance must be in automatic response to the changing pressures of air between the wingtip and the surface of the sea.
All the species of albatross that practice this mode of flight, and the giant petrel as well, may be seen doing the same. It is almost as wonderful to see shearwaters or prions moving swiftly through the avenues between the waves of a tossing sea, remaining below the crests, threading the troughs, skimming the moving slopes—wings bending, dipping, flickering, changing in all their contours and dimensions—yet not once touching the wave surfaces that toss all around them.
I have referred to the wings of birds as if they alone were involved in the performance of flying. But the tail and, in many species, the webbed feet are involved; and, indeed, the whole body of the bird is engaged in the intricate and subtle process.
The delicacy and beauty of bird-flight is the product of long evolution. The quasi-reptilian ancestors of the present birds must have been ludicrous in the long hops they made with the aid of their primitive wings and tails. But evolution is a process of progressive perfection, along different paths for different categories of life. Regarding it as such, we must conclude that these birds have come closer to the ultimate perfection of their being than we have to the ultimate perfection of our own.
In the flight of a bird is a whole philosophy, if only we could read it right.