Amid the High Hills

by Sir Hugh Fraser

With the exception of the difference in the wings mentioned above, the structure of the eagle and falcon is very similar, and as has been pointed out, the larger of two birds of similar structure once it gets going is almost invariably faster, owing no doubt to its superior muscular power and driving force.

In comparing the downward flight of the eagle and falcon it is also necessary to recollect the advantage which the former has by reason of its much greater weight.

It is difficult to obtain thoroughly reliable records of the weights of the golden eagle and the different falcons; but so far as I can ascertain, the weight of the eagle varies from 8½ to 12½ lb., that of the gyr-falcon from 3 to 3¾ lb., and that of the peregrine from 2 to 3 ounces under 2 lb. to 2¾ lb., in each case of course the female bird being heavier than the male.

But for the resistance of the air, all bodies, light or heavy, small or large, would fall at the same rate. In fact, however, as velocity increases a notable air resistance is set up which increases rapidly. The velocity of a body falling freely in vacuo is over forty miles per hour at the end of two seconds, over sixty at the end of three seconds, and so on.

We all know by experience the great force exerted by a wind of a velocity even as low as thirty miles an hour, which most people would call a hurricane. But it is not perhaps so generally known that in proportion to its weight, other things such as shape and specific gravity being similar, a small body experiences much greater resistance than a large body. The resistance of the air to the fine particles of vapour which constitute a cloud is such that they only fall at the same rate of a few feet per hour. And in the case of two birds of similar shape and specific gravity, but one eight times the weight of the other, the larger bird would ultimately attain a velocity roughly twice as great as the other, if both fell for a sufficient distance to attain their limiting velocities, i.e. the velocity at which the resistance offered by the air is equal to the attraction of gravity. Similarly if the one bird were four times the weight of the other, the velocity ultimately attained under the conditions mentioned would be roughly one and a half times as great as the other.

In “Notes by an Old Stalker” in the Field for September 9, 1922 (p. 370) there appears the following interesting account of a duel between a golden eagle and a peregrine which the writer himself witnessed:

“Although by a long way our most powerful bird, the eagle is by no means a match for some much smaller combatants. Once I saw an eagle soaring placidly along when from a range of precipices immediately below him a falcon shot up into the air. Without a moment’s hesitation he attacked the giant bird. The eagle at once joined combat, and through the telescope I could see his efforts to hit his adversary with beak and wing. One blow from either and it would be all over with the falcon; but the latter evidently realised this and regulated his tactics accordingly. The movements of the eagle were slow and cumbrous compared to the rapid action and lithe activity of his adversary. Every time he dodged the eagle’s stroke and, wheeling rapidly, got in his blow before the huge bird could recover himself. That the eagle was in a great rage was evident, for I could hear him emitting sounds that resembled nothing so much as the bark of a terrier. Finally, realising the hopelessness of the contest, he took to flight. I previously knew that the eagle was fast on wing, but the speed he now exhibited was a revelation to me. With half-extended, half-curved wings, showing never a tremor, he cleft the air straight as a bullet. The falcon pursued, but, being left hopelessly behind, soon gave up the chase.”

The flight of the eagle here described was obviously a glide or downward flight, when, as I have pointed out, gravity would assist his speed to a greater extent than it would in a bird of less weight—the peregrine.

In the case of a bird of prey descending from a height on its quarry, the nearer its downward flight is to the vertical the faster will it descend. In coming down on its prey, neither the eagle nor the falcon completely closes its wings, probably because if it did so, it would lose control. This is also true of the gannet or solan goose, which has been described as the largest and noblest-looking of our sea fowl. The great speed which a bird of large size can attain in downward flight can to some extent be realised by watching the gannet when he drops head first as he descends perpendicularly on to the fish in the water. I have carefully examined and compared the skeletons of the eagle and peregrine and have tried to form some idea as to the relative muscular power and driving force of the two birds, and bearing in mind the facts stated above, and the greatly superior size and weight of the eagle, it seems reasonable to conclude on principle that the eagle is probably faster than the gyr-falcon or peregrine in a downward flight, assuming that both birds are putting forth all their powers.