Two hawks come over the trees, and, approaching each other, rise higher into the air. They wheel about for a little without any apparent design, still rising, when one ceases to beat the air with his wings, stretches them to their full length, and seems to lean aside. His impetus carries him forward and upward, at the same time in a circle, something like a skater on one foot. Revolving round a centre, he rises in a spiral, perhaps a hundred yards across; screwing upwards, and at each turn ascending half the diameter of the spiral. When he begins this it appears perfectly natural, and nothing more than would necessarily result if the wings were held outstretched and one edge of the plane slightly elevated. The impulse of previous flight, the beat of strong pinions, and the swing and rush of the bird evidently suffice for two or three, possibly for four or five, winding movements, after which the retarding effects of friction and gravitation ought, according to theory, to gradually bring the bird to a stop. But up goes the hawk, round and round like a woodpecker climbing a tree; only the hawk has nothing tangible into which to stick his claws and to rest his tail against. Those winding circles must surely cease; his own weight alone must stop him, and those wide wings outstretched must check his course. Instead of which the hawk rises as easily as at first, and without the slightest effort--no beat of wing or flutter, without even a slip or jerk, easily round and round. His companion does the same; often, perhaps always, revolving the opposite way, so as to face the first. It is a fascinating motion to watch.
The graceful sweeping curl holds the eye: it is a line of beauty, and draws the glance up into the heights of the air. The darker upper part of one is usually visible at the same time as the lighter under part of the other, and as the dark wheels again the sunlight gleams on the breast and under wing. Sometimes they take regular curves, ascending in an equal degree with each; each curve representing an equal height gained perpendicularly. Sometimes they sweep round in wide circles, scarcely ascending at all. Again, suddenly one will shoot up almost perpendicularly, immediately followed by the other. Then they will resume the regular ascent. Up, like the woodpecker round a tree, till now the level of the rainy scud which hurries over in wet weather has long been past; up till to the eye it looks as if they must soon attain to the flecks of white cloud in the sunny sky to-day. They are in reality far from that elevation; but their true height is none the less wonderful. Resting on the sward, I have watched them go up like this through a lovely morning atmosphere till they seemed about to actually enter the blue, till they were smaller in appearance than larks at their highest ascent, till the head had to be thrown right back to see them. This last circumstance shows how perpendicularly they ascend, winding round a line drawn straight up. At their very highest they are hardly visible, except when the under wing and breast passes and gleams in the light.
All this is accomplished with outstretched wings held at full length, without flap, or beat, or any apparent renewal of the original impetus. If you take a flat stone and throw it so that it will spin, it will go some way straight, then rise, turn aside, describe a half-circle, and fall. If the impetus kept in it, it would soar like the hawk, but this does not happen. A boomerang acts much in the same manner, only more perfectly: yet, however forcibly thrown, the impetus soon dies out of a boomerang. A skater gets up his utmost speed, suddenly stands on one foot, and describes several circles; but in two minutes comes to a standstill, unless he "screws," or works his skate, and so renews the impulse. Even at his best he only goes round, and does not raise his weight an inch from the ice. The velocity of a bullet rapidly decreases, and a ball shot from an express rifle, and driven by a heavy charge, soon begins to droop. When these facts are duly considered, it will soon be apparent what a remarkable feat soaring really is. The hawk does not always ascend in a spiral, but every now and then revolves in a circle--a flat circle--and suddenly shoots up with renewed rapidity. Whether this be merely sportive wantonness or whether it is a necessity, is impossible to determine; but to me it does not appear as if the hawk did it from necessity. It has more the appearance of variation: just as you or I might walk fast at one moment and slowly at another, now this side of the street and now the other. A shifting of the plane of the wings would, however, in all probability, give some impetus: the question is, would it be sufficient? I have seen hawks go up in sunny and lovely weather--in fact, they seem to prefer still, calm weather; but, considering the height to which they attain, no one can positively assert that they do or do not utilise a current. If they do, they may be said to sail (a hawk's wings are technically his sails) round half the circle with the wind fair and behind, and then meet it the other half of the turn, using the impetus they have gained to surmount the breeze as they breast it. Granting this mechanical assistance, it still remains a wonderful feat, since the nicest adjustment must be necessary to get the impetus sufficient to carry the birds over the resistance. They do not drift, or very little.
My own impression is that a hawk can soar in a perfectly still atmosphere. If there is a wind he uses it; but it is quite as much an impediment as an aid. If there is no wind he goes up with the greater ease and to the greater height, and will of choice soar in a calm. The spectacle of a weight--for of course the hawk has an appreciable weight--apparently lifting itself in the face of gravitation and overcoming friction, is a very striking one. When an autumn leaf parts on a still day from the twig, it often rotates and travels some distance from the tree, falling reluctantly and with pauses and delays in the air. It is conceivable that if the leaf were animated and could guide its rotation, it might retard its fall for a considerable period of time, or even rise higher than the tree.
[The end]
Richard Jefferies's essay: Birds Climbing The Air