There has lately been some discussion about the hovering of kestrels: the point being whether the bird can or cannot support itself in the air while stationary, without the assistance of one or more currents of air. The kestrel is the commonest hawk in the southern parts of England, so that many opportunities occur to observe his habits; and there ought not to be any doubt in the matter. It is even alleged that it will go far to decide the question of the possibility of flight or of the construction of an aerial machine. Without entering into this portion of the discussion, let us examine the kestrel's habits.
This hawk has a light easy flight, usually maintaining an altitude a little lower than the tallest elms, but higher than most trees. He will keep this particular altitude for hours together, and sweep over miles of country, with only occasional variations--excluding, of course, descents for the purpose of taking mice. It is usually at this height that a kestrel hovers, though he is capable of doing it a much greater elevation. As he comes gliding through the atmosphere, suddenly he shoots up a little (say, roughly, two or three feet), and then stops short. His tail, which is broader than it looks, is bent slightly downwards; his wings beat the air, at the first glance, just as if he was progressing. Sometimes he seems to oscillate to one side, sometimes to the other; but these side movements do not amount to any appreciable change of position. If there be little or no wind (note this) he remains beating the air, to the eye at least perfectly stationary, perhaps as much as half a minute or more. He then seems to slip forward about half a yard, as if a pent-up force was released, but immediately recovers himself and hovers again. This alternate hovering and slipping forward may be repeated two or three times: it seems to depend on the bird's judgment as to the chance of prey. If he does not think a mouse is to be had, at the first slip he allows himself to proceed. If the spot be likely, or (what is still more tempting) if it is near a place where he has taken prey previously, he will slip and bring up several times. Now and then he will even fetch a half-circle when his balance or impetus (or both) is quite exhausted, and to return to the same spot and recommence. But this is not often, as a rule, after two or three slips he proceeds on his voyage. He will repeat the same round day after day, if undisturbed, and, if the place be at all infested with mice, he will come to it three or four times a day. There is, therefore, every chance of watching him, if you have once found his route. Should he spy a mouse, down he comes, quick but steady, and very nearly straight upon it. But kestrels do not always descend upon prey actually in view. Unless I am much mistaken, they now and then descend in a likely spot and watch like a cat for a minute or two for mice or beetles. For rest they always seek a tree.
Now, having briefly sketched his general manner, let us return and examine the details. In the first place, he usually rises slightly, with outstretched wings, as if about to soar at the moment of commencing hovering. The planes of the wings are then inclined, and meet the air. At the instant of stopping, the tail is depressed. It appears reasonable to conjecture that the slight soaring is to assist the tail in checking his onward course, and to gain a balance. Immediately the wings beat rapidly, somewhat as they do in ordinary flight but with a more forward motion, and somewhat as birds do when about to perch on an awkward ledge, as a swallow at an incomplete nest under an eave. The wings look more, in front, as if attached to his neck. In an exaggerated way ducks beat the air like this, with no intention of rising at all, merely to stretch their wings. The duck raises himself as he stands on the ground, stretches himself to his full height, and flaps his wings horizontally. The kestrel's wings strike downwards and a very little forwards, for his natural tendency is to slip forwards, and the object of slightly reversing his vanes is to prevent this and yet at the same time to support him. His shape is such that if he were rigid with outstretched wings he would glide ahead, just as a ship in a calm slowly forges ahead because of her lines, which are drawn for forward motion. The kestrel's object is to prevent his slip forwards, and the tail alone will not do it. It is necessary for him to "stroke" the air in order to keep up at all; because the moment he pauses gravitation exercises a force much greater than when he glides.
While hovering there are several forces balanced: first, the original impetus onwards; secondly, that of the depressed tail dragging and stopping that onward course; thirdly, that of the wing beating downwards; and fourthly, that of the wing a very little reversed beating forwards, like backing water with a scull. When used in the ordinary way the shape of the wing causes it to exert a downward and a backward pressure. His slip is when he loses balance: it is most obviously a loss of balance; he quite oscillates sometimes when it occurs; and now and then I have seen a kestrel unable to catch himself, and obliged to proceed some distance before he could hover again. Occasionally, in the slip he loses a foot or so of elevation, but not always. While actually hovering, his altitude does not vary an inch. All and each of these movements and the considerations to which they give rise show conclusively that the act of hovering is nothing more or less than an act of balancing; and when he has his balance he will rest a moment with outstretched wings kept still. He uses his wings with just sufficient force neither to rise nor fall, and prevents progress by a slightly different stroke.
The next point is, Where does he hover? He hovers any and everywhere, without the slightest choice. He hovers over meadows, cornfields; over the tops of the highest downs, sometimes at the very edge of a precipice or above a chalk quarry; over gardens, waste ground; over the highway; over summer and other ricks and thatched sheds, from which he sometimes takes his prey; over stables, where mice abound. He has no preference for one side of a hedge or grove, and cares not the least on which the wind blows. His hovering is entirely determined by his judgment as to the chance of prey. I have seen a kestrel hover over every variety of dry ground that is to be found.
Next, as to the wind. If any one has read what has preceded upon his manner of preserving his balance, it must be at once apparent that, supposing a kestrel were hovering in a calm and a wind arose, he would at once face it, else his balance could not be kept. Even on the ground almost all birds face the wind by choice; but the hovering kestrel has no choice. He must hover facing the wind, or it would upset him: just as you may often see a rook flung half aback by a sudden gust. Hence has arisen the supposition that a kestrel cannot hover without a wind. The truth is, he can hover in a perfect calm, and no doubt could do so in a room if it were large enough. He requires no current of any kind, neither a horizontal breeze nor an ascending current. A kestrel can and does hover in the dead calm of summer days, when there is not the faintest breath of wind. He will and does hover in the still, soft atmosphere of early autumn, when the gossamer falls in showers, coming straight down as if it were raining silk. If you puff up a ball of thistledown it will languish on your breath and sink again to the sward. The reapers are sweltering in the wheat, the keeper suffocates in the wood, the carter walks in the shadow cast by his load of corn, the country-side stares all parched and cracked and gasps for a rainy breeze. The kestrel hovers just the same. Could he not do so, a long calm would half starve him, as that is his manner of preying. Having often spent hours in trees for the purpose of a better watch upon animals and birds, I can vouch for it that ascending currents are not frequent--rare, in fact, except in a gale. In a light air or calm there is no ascending current, or it is imperceptible and of no use to the kestrel. Such currents, when they do exist, are very local; but the kestrel's hover is not local: he can hover anywhere. He can do it in the face of a stiff gale, and in a perfect calm. The only weather he dislikes is heavy thunder, rain, or hail, during which he generally perches on a tree; but he can hover in all ordinary rain. He effects it by sheer power and dexterity of wing. Therefore if the fact has any bearing upon the problem of flight, the question of currents may be left out altogether. His facing the wind is, as has been pointed out, only a proof that he is keeping his balance.
The kestrel is not the only bird that hovers. The sparrowhawk can. So can all the finches, more or less, when taking seeds from a plant which will not bear their weight or which they cannot otherwise get at; also when taking insects on the wing. Sparrows do the same. Larks hover in their mating season uttering a short song, not the same as when they soar. Numerous insects can hover: the great dragon-fly will stop dead short in his rapid flight, and stay suspended till it suits him to advance. None of these require any current or wind. I do not think that hovering requires so much strength of wing or such an exercise of force as when birds rise almost straight up. Snipes do it, and woodcocks; so also pheasants, rocketing with tremendous effort; so also a sparrow in a confined court, rising almost straight to the slates. Evidently this needs great power. Hovering is very interesting; but not nearly so mysterious as at least one other power possessed by birds.
[The end]
Richard Jefferies's essay: The Hovering Of The Kestrel