CHAPTER IV.

Modes of Flight.

"The soaring lark is blest as proud When at Heaven's gate she sings: The roving bee proclaims aloud Her flight by vocal wings."--_Wordsworth._

The movements of the wing in flight--Marey's experiments--Stopping and turning movements--Alighting--"Taking off"--Hovering--The use of the tail in flight--The carriage of the neck in flight--And of the legs--The flight of petrels--The speed of flight--The height at which birds fly--Flight with burdens--Experiments on the sizes of the wing in relation to flight--Flight in "troops."

While it is possible to show that certain kinds of flight are to be associated with such and such peculiarities of the skeleton, and the muscles attached thereto, there are many "eccentricities" which cannot be measured, and explained, in terms of mechanism.

The very disconcerting, twisting, flight of the snipe is one of these. The sportsman knows it well: and he knows that the twisting, during which the bird turns the body half over--that is with, say, the left wing pointing directly downwards, and the right wing directly upwards--is only the preliminary to getting fully on the way, and that, presently, it will pursue a straight course, with arrow-like speed. Yet its cousin, the jack-snipe, never twists.

Why does the woodcock invariably drop after a charge of shot, even though not a pellet has touched it, while a snipe pursues its way? These differences are not merely differences of "habit": they indicate subtle differences in nervous response to the same kind of stimulus, and in structural details yet to be unravelled.

Some day the cinematograph will reveal to us all the phases of flight and the movements to which they are due. Even now, thanks to the modern camera, we have learned a great deal. We have learned, for example, that the flight of a bird is not effected merely by rapid up and down movements of the fully extended wings, or with flexed wings--that is to say, half closed, as in "gliding" flight when a bird is descending, or in the swoop of, say, the sparrow-hawk. Only in one of these two positions do we ever seem to see the wings when we have to trust to our eyes alone, as the bird hurries past us. The impression that we have seen aright is confirmed when we stand on the deck of a steamer, and watch the gulls following in its wake. For incredibly long distances they will travel without a perceptible wing-beat. The albatross is the finest of all performers in regard to this kind of flight, which is due, apparently, to air currents created by stiff breezes, or gales. Some birds seem to make their way against a head-wind with the minimum of effort, by partly flexing the wings and gliding downwards: at the end of the descent, by turning the body sharply upwards, and spreading the wings to the fullest extent, they are lifted up, and driven forward, like a kite.

Marey and Pettigrew, long ago, showed conclusively, by means of photography, that our conception of the movement of the wing during flight was far from correct.

To avoid a long and tedious description, and many technicalities, it must suffice to say that the wing of a bird possesses very considerable freedom and range of movement at the shoulder joint. Certainly, during some phases of flight, the wings are thrust forward and extended to their fullest extent, so that the outer margins of the wings come to lie almost parallel with the long axis of the body, as may be seen in the spirited illustration showing the goshawk in flight. As they sweep downwards, and backwards, they lift the body and drive it forwards. At the end of the "sweep" they are "flexed," that is to say, bent at the elbow and wrist-joints, while at the same time they are raised and brought forward above the body for a repetition of the stroke. These movements are too quick for the eye to follow, but they have been fixed for us by the camera.

Marey devised an ingenious experiment in his endeavour to discover the movements of the bird's wing during flight. He fastened a small piece of paper to the tip of a crows wing, and as the bird flew in front of a perfectly black screen he took a photograph of this moving speck of white, while, of course, no image of the crow appeared on the plate. The resultant picture gave a series of "figure of 8 loops" as one would make this figure with a pen, contriving to make the lower loop very small, and the upper loop very large. But as the wing-beat increased in speed the lower loop gradually faded out.

These movements of the wing, however, are descriptive rather of what takes place during very vigorous flight, as when the bird is getting up "steam." When he is well under way there is no need for these long and very tiring strokes, except in the case of birds like the pheasant or the duck. A gull, when in full career does not, apparently, raise the wings very high, nor depresses them very low, nor does it flex the wings at the wrist-joints.

Stopping and turning movements are generally extremely difficult to follow, because they are performed so quickly. They can be seen fairly easily in the case of some of the larger birds. Ducks, as is well shown in one of our coloured Plates, draw the head backwards, tilt the body upward, thrust the feet forward, and spread the tail, at the same time turning it forwards. Gulls and pigeons too may be watched with profit.

In turning, the body is tilted sideways, so that the tip of one wing points skywards, the other earthwards, as in the case of the goshawk illustrated in this book. The pigeon, and some other birds seem further to spread out the long, stiff quills borne by the thumb, which form what is known as the "bastard-wing." This turning movement is well shown, again, in the very realistic coloured picture of the woodcock turning in mid-air, and bearing too the burden of one of its nestlings.

If it is difficult to satisfy oneself as to the way in which a bird alights, it is no less so to detect its movements in taking wing. Most of us must have seen sparrows making this effort from the road, thousands of times. But ask of anyone, How is it done? The act takes place so quickly that the eye cannot follow its execution. And what is true of the sparrow is true of most birds. But there are some where this is not the case. Many water-birds, the cormorant, for example, get under way but slowly, and with evident effort. They flap along the surface for some distance before they gain sufficient impetus to lift them into the air. And there are many long-winged, short-legged birds which can rise from a level surface only with great difficulty, or not at all. The swift is one of these, for its legs are excessively short. The albatross is another: and this is true, indeed, of many of the petrel-tribe. The puffin, again, seems unable to rise on the wing from the ground. It appears invariably to run along until it reaches the edge of cliff which lodges its burrow, and then, as it were, throw itself over the edge. The heron, when springing into the air, stretches his long neck out to its fullest extent, and presents a pair of dangling legs, well shown in one of our coloured Plates, but when once fully on the way its pose entirely changes, the neck being drawn in and the legs thrust out backwards.

Flight does not always mean progress through the air. Most birds can, at need, arrest their course, and hang, as it were, suspended in the air. In the beautiful coloured plate, representing the chaffinch hovering over its half-fledged young, and in that of the kingfisher and its young, this form of "hovering" flight can be seen. But the greatest of all exponents in the art of hovering is the kestrel, known also, for this very reason, as the "windhover." It is most fascinating to watch this bird hang, as it were, from the clouds, motionless, yet with quivering wings, as he scans the ground below in his search for some unsuspecting mouse. It is hard, indeed, to say which is the more wonderful, this power of remaining stationary for comparatively long periods in the air, or the surprising powers of sight which this bird possesses. During these hovering movements, always head to wind, it will be noted, the tail plays a very important part, being spread to its extremest limit, and at the same time thrust forward beneath the body. In some birds this forward movement is more marked than in others. And this because such birds possess a somewhat more flexible spine, there being a certain amount of "play" where the vertebræ of the loins join the welded mass of vertebræ which lie between the bones of the hip-girdle.

But the tail feathers are not indispensable. This much is shown in the case of birds like the kingfisher, the water-hen, and the land-rail, which contrive to fly well, and at a great pace, though they have but the merest apology for a tail. More than this, the grebes have no tail at all. But it is to be noted that they are by no means adept at turning movements; owing to the lack of this appendage the body, when in mid-air, has a curiously truncated appearance, as may be seen in the illustration. Further, it is significant that in the contemptible "sport" of pigeon-shooting from traps, the birds are deprived of their tails to prevent them from making turning movements.

The carriage of the head and neck, and of the legs, during flight presents some interesting, and some instructing contrasts.

Ducks, geese, and swans, flamingoes, storks, and cormorants always fly with the head and neck stretched out to their fullest extent. Herons and pelicans, though also long-necked birds, draw the head back till it rests almost on the shoulders. Most birds, indeed, fly with the head drawn back towards the body. The appearance of some of these birds on the wing can be seen at a glance on turning to the page illustrating this aspect of flight.

Not so very long ago a great controversy was waged as to what birds did with their legs during flight. Many of the older artists invariably depicted them drawn up under the breast. But as a matter of fact, this method seems to be confined to the Passerine birds--the "perching birds," such as crows and finches and their kin. It has yet to be settled what obtains among what are known as the "Picarian" birds, such as kingfishers, bee-eaters, woodpeckers, and so on. The legs and feet of these birds are so small, and their flight is so rapid, that the matter is by no means an easy one to settle. But all other birds carry the legs and toes bent backwards, under the tail. In the gulls, this can easily be seen, and easier still in the case of the common heron, where they are, as it were, trailed out behind--owing to the shortness of the tail and the great length of the leg. The puffin carries them "splayed" out on each side of his tail, and so also do his kinsmen, the razor-bills, and guillemots.

The legs, as a rule, take no part in flight. True, they can be seen thrust out just before alighting, but this is solely for the purpose of effecting a safe landing. But where gulls can be watched at close quarters, as in harbours, round a ship, or in such favoured spots as are to be found about the bridges of London during the winter, careful watch will show that the legs are frequently used when efforts are being made to turn, or check the speed of flight.

Some of the smaller petrels--like the storm-petrel, or "Mother Carey's chickens," will patter over the water with their feet as they fly just over the surface of the waves.

Whether the legs are carried drawn close up beneath the breast, or thrust backwards under the tail, the purpose of this disposal is the same--to prevent any interference with the "stream-lines" of the body which would impede flight.

On the matter of the speed of flight there seems to be much misconception. Gätke, the German ornithologist, gravely asserted that the little Arctic blue-throat--one of our rarer British birds--could leave its winter resort in Africa in the dusk of evening, and arrive at Heligoland--where he spent so many years studying bird migration--nine hours later. That is to say it could travel 1,600 geographical miles in a single night, at the astounding velocity of 180 miles an hour! According to another estimate of his, curlews, godwits, and plovers crossed from Heligoland to the oyster-beds lying to the eastward, a known distance of rather more than four English miles, in one minute; or at the rate of over 240 miles an hour. Against such extravagant estimates it is hardly necessary to bring rebutting evidence. But if any be demanded it may be furnished by the carrier pigeon, which has been known to maintain a speed of 55 miles an hour for four hours in succession: and it is extremely unlikely that this is much, if at all, exceeded by any wild bird during long-distance flights.

That our spring and autumn migrants must possess wonderful powers of endurance is beyond question. And it is equally certain that thousands must perish by the way. By this means is the standard of flight maintained--the weak perish. Even the minimum standard of efficiency for the survival of such an ordeal must be a high one.

Few of us see anything of these marvellous migration flights. For, in the first place, they are generally performed at night, and at a great height, often beyond the range of human vision. Only as they approach land, and their destination, do they descend. American naturalists have made some interesting observations by directing a telescope against the sky. Thus, Mr. Frank Chapman, by turning his instrument towards the full moon, has seen birds passing at night at an altitude, according to his computation, of five miles: while the late Mr. W. E. D. Scott saw, through an astronomical telescope at Princeton, New Jersey, great numbers of birds passing across the face of the moon--warblers, finches, and woodpeckers among them. Mr. Chapman again, on another occasion, saw no less than 262 birds pass over the field of his telescope at a height of from 1,500 to 15,000 feet: and the most remarkable thing of all was the fact that the lowest birds were flying upwards, as if they had risen from the immediate neighbourhood and were seeking the proper elevation to continue their flight.

As has already been remarked, when nearing their destination migrating birds descend, though still many miles from land. Should a gale be raging they fly so low that they barely top the waves. And this, apparently, to escape, so far as is possible, the force of the wind. Larks, starlings, thrushes, and other small birds, can sometimes be seen during daylight crossing the North Sea in their thousands. At such times many will often afford themselves a brief rest in the rigging of ships, homeward bound, but the main host hurry on. The beautiful golden crested wren, our smallest British migrant, is one of these. A glance at our charming coloured plate will show at once that the wing is not that of a bird of strong flight. There is no more interesting experience to the bird-lover than that of watching the tired travellers drop earthwards, as they leave the dreadful sea behind them.

With all birds yet retaining the power of flight there is always a liberal "margin of safety" in regard to the wing area. That is to say this is always in excess of the minimum area necessary to make flight possible. This much, indeed, is manifest from the fact that the eagle can bear off a victim equalling himself in weight. Should he miscalculate, he can always drop his burden, or lessen its weight by eating part of it on the spot. Not so the osprey, or the sea-eagle, which have been known to plunge down and drive their talons into fishes too large to be raised. Unable to release their grip, death, by drowning, has inevitably followed.

Sometimes the burden is a passenger, instead of a victim. One of the most striking of the coloured plates in this volume is that of a woodcock carrying one of its nestlings to a distant feeding place. This habit is well known. It is not often that the necessity arises, but there are occasions where suitable nesting and feeding grounds cannot be found together, or when, as during prolonged drought, the normal feeding area dries up. Then, instinctively, the parent will surmount the dangers of starvation for their offspring, by conveying them to a land of plenty, returning again to the shelter of the wood as soon as the meal is over. The weight of a newly-hatched nestling, it is true, could scarcely be called a "burden." But they are carried about thus until they are strong enough to perform the journey for themselves. Thus, then, towards the end of the nursing period the weight to be carried is by no means a light one.

But it was shown, long since, by direct experiment, that the area of a bird's wing is considerably in excess of what is required for the purpose of flight. Dr. J. Bell Pettigrew, more than fifty years ago, to test this matter, cut off more than half of the secondary wing feathers of a sparrow, parallel with the long axis of the wing. He first clipped one, then both wings, and found that in both cases flight was apparently unimpaired. He then removed a fourth of the primary feathers--the outermost quills--and still the flight was unimpaired. At any rate the bird flew upwards of thirty yards, rose to a considerable height and alighted in a tree. Thirty yards, however, is a short flight even for a sparrow. But it is enough to show that flight, if not _sustained_ flight, was possible after this mutilation. Not until more than one-third of the quills along the whole length of the wing were removed, did the flight become obviously laboured. And he found that what was true of the sparrow, was equally true of the wings of insects.

Though these experiments demonstrate, in a very unmistakable manner, that flight with a greatly reduced wing area is possible, we have no evidence that this reduction would make no difference to the length of time the bird could remain on the wing. And this is a very important matter.

An aspect of flight which has now to be considered is that of birds which fly in troops. Some species always travel thus, others only on occasions. Rooks and gulls afford instances of this, when, during windy weather, or for other reasons, they congregate and fly round and round in great circles, at a considerable height. Small wading-birds, like ringed plovers and dunlin, commonly fly in "bunches." The last named furnish a singularly interesting sight when thus travelling; for their evolutions are so amazingly timed. As if at a given signal every bird in the troop will change its course at the same moment, and in the same direction, so that now one sees a flickering mesh-work of grey, and now a shimmering as of snow-flakes, as first the grey backs, and then the white breasts are turned towards one. But flights such as this are to be seen only during the autumn and winter months. For during the breeding season these little flocks are broken up and distributed far and wide. But there is yet another reason. They wear a totally different dress--the courtship or breeding plumage. Herein the upper parts are of a rich chestnut hue, streaked with black, while the under parts are black. Even more fascinating to watch are the autumn troops of starlings on the way to their roosting places. Hundreds at a time, not to say thousands, take part in these flights. Now they rush onward, in one great far-flung sheet, and now they close up into a great, almost ball-like, mass: and now they thin out till they look like a trail of smoke. But always they wheel and turn and rise and descend, not as separate bodies, but as one. How are such wonderful evolutions timed. The movements of an army on review-day are not more precise, or more perfectly carried out. During the whole flight not a sound, save the swishing of their wings can be heard. The marvel of it all is beyond the range of words, nor can one express the peculiar delight such a sight affords.

Why is it that ducks and geese commonly fly either in Indian file, or in a roughly V-shaped formation, with the apex of the V forward? Why do they not fly all abreast? One cannot say, but they never do.

Some mention must be made here of the surprising numbers in which geese, of some species, congregate. Writing of the Brent goose, in his "Bird Life of the Borders," Mr. Abel Chapman--and there are few men who can write with such authority on the subject--tells us:--"Just at dark the whole host rise on the wing together, and make for the open sea. In the morning they have come in by companies and battalions, but at night they go out in one solid army; and a fine sight it is to witness their departure. The whole host, perhaps ten thousand strong, here massed in dense phalanxes, elsewhere in columns tailing off into long skeins, V's or rectilineal formations of every conceivable shape, (but always with a certain formation)--out they go, full one hundred yards high, while their loud clanging, defiance--"honk, honk,--torrock, torrock," and its running accompaniment of lower croaks and shrill bi-tones, resounds for miles around."