Art. XVI.—On the Flight of the Black-backed Gull (Larus dominicanus).
[Read before the Auckland Institute, 19th August, 1872.]
The phenomenon of flight has of late years attracted considerable attention, and the subject has been very fully and ably discussed, especially by Dr. Pettigrew, of London (Trans. Lin. Soc., 1868, p. 197), and Professor Marey, of Paris (Smithsonian Report, 1869, p. 226). Both these authors have been very successful in explaining the flight of insects, but considerable obscurity seems still to exist as to the actual movements of the wings of birds when flying. Mr. Macgillivray (British Birds, Vol. L., p. 34) said that the effective stroke of the wing is delivered downward and backward, and suggested that during the down stroke the resistance of the air bends upward the free tips of the feathers, and the reaction thus produced gives a forward impulse to the bird. The Duke of Argyll (Reign of Law, p. 132, 1867) and Professor Marey both hold a similar view, but while the former maintains that the effective stroke is delivered directly downward, the latter says that his experiments prove that during the down stroke the wing moves first slightly forward, then more and more backward; and in the up stroke at first backward, and then forward into its original position again. Dr. Pettigrew on the other hand asserts that the effective stroke is delivered downward and forward, and that by a peculiar twisting or screwing motion of the wings, which I confess I do not quite understand, the air is forced to escape near the root of the pinion, between the secondary and tertiary feathers, in a downward and backward direction, thus by its reaction supporting the bird and driving it forward.
Professor Marey again says that during the greater part of the down stroke the wing, by turning on its axis, slopes forward and downward, while during the up stroke it slopes forward and upward, thus being on this point quite opposed to Dr. Pettigrew, who states distinctly that during the down stroke no depression of the anterior margin and elevation of the posterior one takes place. Dr. Pettigrew, and the Duke of Argyll also, both say that during flight the point of the wing describes a “wave track,” or simple undulating line through the air, while Professor Marey says that his experiments show conclusively that it describes a more or less regular cycloidal curve, or looped line. All four authors, however, agree that the wing is extended during the down stroke, and more or less folded during the up stroke. Under these circumstances a few observations that I have made on the movements of the wings of the common black-backed sea-gull during flight may prove of interest, for not only do they point to a theory of progression much simpler than any
hitherto proposed, but they also supply an explanation of many of the differences between other authors, and this curiously enough by showing that they are mistaken in the only point on which they are all agreed, viz, the folding of the wing during the up stroke.
Before, however, describing my observations, I will mention some of the very interesting experiments made by Dr. Pettigrew on the flight of sparrows, with their wings cut in different ways, which, in my opinion, not only annihilate, as he says, Mr. Macgillivray's theory, but his own also.
From his experiments I pick out the following as the most decisive.
1. Half of the secondary feathers of both pinions detached in the direction of the long axis of the wing, the primaries being left intact. Result.—Flight perfect.
2. Half of the primary feathers in the long axis of either pinion detached, the secondaries being left intact. Result.—When one wing only was operated on flight was perfect, when both were cut it was slightly laboured.
3. Primary and secondary feathers from both wings removed alternately. Result.—Flight nearly perfect.
4. Half the primary feathers from either wing removed transversely. Result.—When one wing only was operated on flight was but very slightly impaired, when both were cut the bird flew heavily and came to the ground at no great distance.
These experiments prove that cutting the wings in the direction of the long axis interferes very little with flight, but that if the tips of the primaries are cut off transversely the effect is very evident. This, in other words, means that flight depends principally on the primary feathers of the wing, and not on the secondaries, while both Mr. Macgillivray's and Dr. Pettigrew's theories imply quite the reverse, for the former says that progression is obtained by the uplifting of the secondary feathers, and the latter by the secondary feathers forming a kind of funnel which compels the air to escape in a backward direction. Dr. Pettigrew himself (l.c., 245) says that “the bending up of the shafts of the feathers during the descent of the wing would impair its efficiency by permitting more air to escape along its posterior, or thin margin, than is necessary;” much more, therefore, ought its efficiency to be impaired by cutting off the shafts of the feathers. But experiment proves clearly that such is not the case.
There is no better time for observing the movements of the wings of a bird than when at sea, steaming against a fresh breeze, and surrounded by a flock of sea-gulls. Under these circumstances the birds often appear to be quite stationary, sometimes straight overhead, sometimes astern, and sometimes on one or the other quarter, so that distinct views from below, from the front, or from one side, can be obtained, while the movements of the wings of
the gull are so slow that the eye can easily follow them. An attentive examination will convince anyone that the wings are moved from the shoulder straight up and down, or very nearly so, that the elbow joint is not appreciably bent during either stroke, but that during the down stroke the wrist joint, which bears the primary feathers, is bent back, and expanded again during the up stroke. While, therefore, the movement of the main part of the wing from the shoulder is nearly vertical, the tips, by having also a horizontal movement, do not describe a simple “wave track” in the air, but a cycloidal curve as stated by Professor Marey.
I should, not, however, omit to mention that Dr. Pettigrew, who also says that flexion occurs principally at the wrist joint, states that while watching rooks he has, over and over again, satisfied himself that the wings are flexed during the up stroke. The rook, however, cannot be compared to the gull in affording facilities for observation. It cannot be seen so near, it moves its wings faster, and it never occurs under those circumstances just mentioned, when the bird, although flying through the air, appears to be stationary, sometimes for more than a minute at a time. Still, I must allow that confirmatory evidence is necessary to others before they can accept my statement as correct, while at the same time such evidence would be very satisfactory to me.
If, however, I am correct in stating that this backward, or rowing motion of the primaries, is delivered during the down stroke, it is obvious that it is this that drives the bird forward, easily, therefore, explaining the results arrived at in the previously mentioned experiments, viz.—that when the primaries are cut flight is stopped, but that when left intact it is but little impeded, although the secondaries are cut off.
It is also obvious that, in order to preserve a steady line of flight, it will be necessary to expose a greater surface of the wing to the air while it is being raised than while it is being depressed, in order that it may support the bird by its kite-like action, as I have explained in my previous paper on the flight of the albatros (Trans. N.Z. Inst., II., 230). The truth of this has been proved by the experiments of Professor Marey, who has shown that during each complete vibration of the wings, a bird rises and falls twice successively, but that these oscillations are unequal in extent, the greater corresponding to the depression of the wings, and the lesser to their elevation; this latter being caused by their kite-like action just described.
From an anatomical examination of the wing, Dr. Pettigrew states that “during flexion the anterior margin is slightly directed downwards, and in extension decidedly directed upwards.” This is just what we should expect if flexion takes place during the down stroke, and it will then agree with Professor Marey's experiments; and it is, I think, entirely from supposing that flexion must necessarily occur during the up stroke that has led Dr.
Pettigrew to the extraordinary opinion that the forward movement of a bird is derived from a stroke delivered downward and forward.
Dr. Pettigrew, and many other authors, hold the opinion that the wing feathers of a bird open and close during the up and down strokes respectively. But however this may be with birds that only flap their wings slowly, it is, I think, almost impossible that such rapid changes should take place in the wings of a bird like the sparrow, which, according to Professor Marey, makes thirty-three vibrations per second. Dr. Pettigrew's experiments, also, upon the sparrow, with alternate feathers taken out of the wing, show that an opening and shutting movement is not necessary for flight; and we may safely assume on the principle of greatest economy of force, a principle always acted upon throughout nature, that what is not necessary is not used.
The falconers of olden days were well aware that rapidity of flight depended on the primary feathers of the wing, and they called these the “flight feathers,” while the secondaries they called the “sail feathers,” and it will be found that the swiftness of a bird's flight depends on the length of the primaries in proportion to the size of the bird, and on the number of strokes it makes per second. Thus the swift, which has proportionately longer primary feathers than any other bird, is probably the fastest flier, while the partridge, which has broad wings but short primaries, flies heavily, and has to make very rapid strokes. The wild-duck has less area of wing in proportion to its weight than a partridge, but its primaries are longer, and consequently it flies much faster. The landrail also is another example of a slow flying bird with considerable expanse of wing for its weight, but with short primaries. The heron also furnishes another instance of the same kind, and it is well known that the long winged falcons are far superior fliers to the round winged buzzards, vultures, and eagles, although in the latter the area of wing surface is probably greater than in the former.
The way in which birds turn in the air has also been much misunderstood. Professor Owen (Comp. Anat. of Vert. II., 115) advances the extraordinary theory that when a bird wishes to turn it beats the air more rapidly with one pinion than with the other, which however originated with Borelli in his “De Motu Animalium.”
Van der Hoven (Handbook of Zoology, II., 371) also reiterates the same opinion, while Macgillivray (l.c. I., p. 420) says that turns are effected by the contraction of one wing and the extension of the other, aided by the tail.
The real method of turning, however, is very simple, and was, I believe, first pointed out by me in the Ibis for July, 1865, p. 297. It must be remembered that when a bird is flying the reaction of its wings against the air is not only forward but also upward, the latter being necessary to counteract the force of gravity. If now a bird lowers its right side, so that the axis from
the breast to the back, which was before perpendicular, is now inclined to the right, part of the upward reaction will be diverted to the right, and will therefore turn the bird in that direction. Of course the force thus diverted will be taken from that necessary to counteract gravity, so that the bird would fall if it did not compensate for this loss by increasing the angle to the horizon at which it was flying. So that if a bird wishes to turn to the right all it has to do is to elevate the left and lower the right side of the body, and at the same time elevate the fore and lower the hinder parts of the body; if it wishes to turn to the left, it will elevate the right and fore parts, and lower the left and hind parts, and the sharpness of the turn will depend entirely upon the angle that the wings, or rather the line drawn from tip to tip of the wings, makes with the horizon. This movement may be easily seen in the pigeon, gull, pheasant, or indeed in almost any bird.