The Mystery of the Soaring Hawk
‘FOUR things are too wonderful for me,’ said the writer of the last chapter of Proverbs. This was one of them: ‘The way of an eagle in the air.’
What lover of nature has not wondered to watch a hawk circling on motionless wings in the blue above him? Round and round he goes, soaring till he is but a speck in the sky. How can a bird raise its weight against gravity without visible effort? What power sustains the hawk?
Explanations have been offered —childish, contradictory, unscientific explanations. The hawk ‘floats’; he ‘sails’; he ‘flies like a kite’; he ‘rises on ascending currents of air’; ‘though the wings as a whole are motionless, the individual feathers are working.’
Let us first dispose of these theories, and then proceed to find the true solution of the problem.
The hawk cannot ‘float,’ for he is heavier than the air he displaces. His quills and bones are full of air. As well might you expect a submarine to float because it was full of water. The lightness of the hawk’s structure gives him less weight to lift; but were he full of hydrogen, he would not float. Shoot him: he will fall to earth with a thud.
Does the hawk ‘sail’? He cannot. The resistance of the water against a boat’s keel or centreboard holds it to the wind, and a resultant of the triangle of forces which act upon it gives it its forward motion. A boat without keel or centreboard cannot sail against the wind. Lacking a grip on the water, it is blown to leeward. Imagine a skiff so light that it floats absolutely on the surface: whatever the angle of its sails, it will be driven down wind like a leaf or a feather. Therefore the hawk, who has no keel in water to hold him to his course, may be blown down wind, but can never sail across or against the wind, let him trim his pinions how he will.
Observe, too, how seagulls fly with motionless wings right in the wind’s eye — a course no racing yacht can follow, for all its deep keel and spread of canvas. This is not sailing. There is power here, and independence to defy the wind. We must seek elsewhere for its source.
What the keel is to the boat, its string is to the kite. Cut the string, and the kite will be blown down wind till gravity brings it to the ground. The hawk has no string. He does not ‘fly like a kite.’
Jefferies, in his Life of the Fields, suggests that the added velocity gained in making one half of the circle down wind is sufficient to bring the hawk back against the wind. This theory is mechanically unsound, and may be commended to seekers after perpetual motion.
We finally dismiss all these windtheories of flight with the observation that hawks prefer a still, windless day for soaring.
Does the bird avail himself of ascending air-currents? On a still, hot day you may see little whirls of dust rising straight up from the ground, sometimes to a great height. But it is inconceivable that the hawk, keeping his regular circles without check or break, could find ascending air-currents to sustain him at every point of his unfaltering flight. A bird weighing two pounds is no more immune from the attraction of the earth’s mass than two pounds of lead or pig-iron. The theory is inadequate to account for the support of a single feather.
‘Though the wings as a whole are motionless, the individual feathers are working.’ Here, at least, is an honest attempt to find some force to counteract the force of gravity. But t he theory is not borne out by observation. All the evidence is against it. Charles Dixon, in British Seabirds, writes: ‘That these flights arc accompanied by any vibratory movements of the feathers is erroneous, as I have had many opportunities of satisfying myself, especially when observing the flight of the fulmas at St. Kilda, the birds not being more than six feet away from me, when I am positive every individual feather was in perfect rest.’ Anatomy would lead us to the same conclusion. There is no muscular structure for such a method of flight, and the wing-feathers are ingeniously felted together to work as a whole. We can dismiss that theory also.
Now let us apply our common sense to the solution of the problem.
Here we have a force, the force of gravity, acting on a body in mid-air. That force must produce a downward acceleration unless it is balanced by some equal and opposite force. Therefore we start with the hypothesis that the hawk must exert force equal and opposite to that of gravity, if he is to maintain his altitude, and still more force to increase his altitude. How does he apply that force? What mechanism has he? A pair of wings, and the big motor-muscles on his breast. We may safely affirm that he sustains his weight by using this mechanism.
‘But the hawk’s wings are motionless when he soars.’
You will admit, then, that, when he flies with beating wings, he is lifting his weight by the use of those breastmuscles? And yet you believe that the bird, which must work those powerful muscles in ordinary flight, ceases to exert them when he spreads his wings wide and starts to soar? Has he lost his weight? He must continue to exert them. There is no alternative. All other theories have been quashed.
How is this possible, when the wings are motionless and not even a feather is vibrating? That’s the real question we have to answer.
To clear our minds of a very natural error, let us consider first the principles of ordinary flight. You think that a bird beats his wings down through the air when he flies? You are wrong. Wings are not made to be driven down through the air. They arc spread and shaped and woven to encounter maximum resistance; to lean on the air, not to cut through it. Between each two strokes there is a fall of both wings and body, due to gravity. The fall of the body is neutralized by the lift of the wing-beat; the fall of the wings is not neutralized; they are now below the body and must be raised for another stroke.
This principle will be understood more readily if we compare it with the motion of a boat’s oars. Those oarblades appear to be driven through the water in a wide arc; in reality they do not move six inches. A little swirling eddy marks the spot where each oarblade has rested throughout the stroke. First, the blades drive the boat forward; then they recover their position for a new stroke. If now a boat is being rowed against the stream and making no headway, and if you cannot see the flow of the stream, but only the motionless boat and the swinging oars, you will find it hard to believe that those oar-blades are not moving through the water. Change the plane from horizontal to vertical, with gravity to represent the sweep of the stream, and it will be clear that each wing-beat lifts the body; it does not drive down the wings.
The resisting medium is not nearly so dense, and the wing-surface is proportionately greater. A heavy-bodied domestic fowl does sometimes drive its wings down in a vain attempt to fly —just as you can tug your oar-blade through the water by putting your boat’s nose against the bank; but a broad-winged, light-bodied hawk finds ample support.
The point to remember is that the wing-force may be exerted without driving down the wing.
Now, we have seen a hawk flapping over the tree-tops, and we know how he looks when he is using both wings together in ordinary flight. When he starts to soar, knowing as we do that he must still be using the same mechanism, we are bound to admit that he is using it in a different way. If not both together, in what way is he using his wings? He is using them alternately.
‘But surely, if that were so —’
You think one wing would be pointing downward and the other sideways, and then vice versa? That is why we discussed the principles of ordinary flight, so that it might be understood that the wing is not driven down. Picture to yourself the hawk, with wings wide for soaring. What will be the result when he contracts the breathmuscle on one side only? One of two things must happen: either the wing must be driven down, or the body must be drawn upward and sideways toward the wing. Which is the more reasonable, to suppose that the broad pinion would be driven down through the resisting air, or that the body would be tilted toward the wing? Emphatically, the latter.
The action of the corresponding muscle on the other side brings the body back to normal and up toward the other wing. Thus a downward pressure of each wing is exerted, with a force equal and opposit e to that of gravity, and the only visible motion is a slight swaying of the body.
You will ask, why has this swaying motion of the body not been noted by observers? It has been noted, but the right deduction has not been made. It has been attributed to balancing. Seen from below, — and the hawk is usually above us, — it is not conspicuous. The wide spread of the wings holds the eye, to the exclusion of minor details. Much more easily one may observe the swaying motion of seagulls following a ship.
In the alternate wing-beat, then, we find the solution of our problem. The hawk does not cease to exert himself: he simply changes his gait. He prefers the smooth motion of the pacer to the jolting trot of the saddle-horse. The soaring hawk is using his adequate strength with the ease and grace of an athlete who obtains the greatest results with the least visible expenditure of effort.