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The kite, the sparrow and the buzzard the wind for their movement. Simple practice the kind of flight called the observation permits us to establish among sailing flight, but the true kings of the birds some clearly defined classes: air are the eagles, the vultures and the First, birds exclusively rowers ; second, condors which travel through space with- birds practicing the rowing flight and out a single beating of the wings like soaring flight; third, birds practicing living aeroplanes, capable even of re- rowing and sailing; fourth, birds pracmaining immovable, as though fastened ticing only sailing flight. in the sky.
Are these distinctions, which
, Even superficial observation enables based only on the different variety of one to ascertain that the same bird may methods for accomplishing the action of have different modes of fight and that flying, dependent upon the construction the different species fly in different ways. of the wing, and are there found in the
Two kinds of flying are in fact ad- wings of a bird such modifications that mitted; that obtained exclusively by the we may without hesitation attribute to beating of the wings, the flight by row- them the special character of the flight? ing, as it may be called, and the soaring Observers have always replied in the flight, during which the bird keeps its affirmative, after having proved that the wings extended in the course of its form of the wings is essentially variable progress through the air. There are two and adapted to the kind of flying ; but it different modes of soaring flight; soaring is modern research which has scienflight properly so called, which is only tifically established this dependence.
There exist two well defined. types of wings among which, evidently, all the gradations are observed, but the distinction is easy. It is sufficient indeed, to glance at the two figures five and six in order to discover that the hawk's wing is stretched to a point, while the eagle's is rounded. This is due to the length of the quili feathers which diminishes from the first one to the last in
temporary and accessory to rowing flight, and sailing flight, which is the normal mode of locomotion for large birds capable of remaining entire days in the air by
find their prey.
the hawk's wing, while in the eagle's have a more ample flight, possess narwing the longest feather is the sixth. rower wings which, in addition, facilitate Besides, the rowing wing is homogeneous their veerings. The wing stretches in on its posterior side, whilst the sailing proportion as the bird, from being a wing is jagged. This last peculiarity is rower, becomes a sailer. But here again due to the special conformation of the one finds some broad and some narrow primary feathers which, instead of pre- wings. The latter, provided for work senting the form of
in violent winds, a knife as do the
belong to sea birds, rowing feathers,
such as the petrel become narrow at
and frigate birds; the middle of their
the others have length while the
been given to the quill loses some of
great birds of prey its rigidity. There
in order that they results certain
may take advansuppleness of the
tage of the least edge of the sailing
breath of wind wing which, during Fig. 4. APPEARANCE OF A Bird THAT SOARS SLOWLY. and may cover fligit, becomes conThe point of the wing carried forward.
enough space to vex and appears indented by reason of the divergence and Now, how does the bird use its wings the bending of the feathers. This atti- in order to keep itself up and to proceed tude is striking in figure ten, and it is pre- through the air ? Flying always comcisely this lack of rigidity which renders prises three phases or periods; the desailing birds unfit for rowing flight. parture, the flight, properly so called, and
Besides these characteristics, based for lastly the alighting. We are going to the most part on the structure of the study these three phases in each one of wing, one may again examine the rela
the groups of birds that we have detion of the two dimensions, length and scribed: rowers, half sailers, and sailbreadth. According to the French Mou- ers. illard, who was one of the most careful The average species of small size, beobservers, the rowers all have short longing to the rowing group, take their
wings but the start after a hop
the wings which
The sparrow, the to traverse by one quail, the partridge, flight but
but they gallinaceous birds, must have power- and pigeons rise in ful wings in order this way. Certain to rise from the aquatic species with ground and es- short wings do not cape danger as need to hop in swiftly pos- order to release sible:
their their wings; for wings are short some ducks it is and broad. The sufficient to
duck and the straighten their Fig.'5. THE ROWING WING
Fig. 6. THE SAILING WING pigeon, which bodies vertically so
OF A FALCON.
OF AN EAGLE.
as to permit at the first a flapping of wings. When these birds rest on an elevated support, it is sufficient for them to let themselves fall in order to acquire the start or indispensable spring for the action of the wings.
The necessary effort for flight is considerable, but it diminishes rapidly in proportion as the speed of the bird approaches the normal condition. One gains an understanding of this by
Fig. 7. THE FRIGATE BIRD-ONE OF THE STRONGEST OF FLYERS. appreciating the speed and amplitude of the flapping of the wings; in the sea gull the amplitude components—the vertical and the horiattains one hundred to one hundred and zontal; the first serving to raise the wing ten degrees at the departure and lowers and the other, directed with a contrary from thirty degrees to forty degrees in motion, consequently slackens the speed normal flying ; in the partridge the ex- of the bird. When to the relative wind penditure of force is so depressing that there is added the absolute wind,—that the little cries that this bird makes at the is, when the bird fies against the wind, moment of its flight are attributed to —the result is still more defined, and this fatigue.
explains why many rowing birds try During the flight the raising of the always to take their flight with the beak wing is obtained only by the action of toward the wind. the middle pectoral muscle which has no All authors are agreed in defining other function and acts intermittently; soaring as a word which signifies that but this would be insufficient if nature kind of Aight which a bird executes withdid not aid by an artifice to reduce the out flapping the wings, and with the resistance of the air to its lowest value. wings more or less extended. Soaring It has been discovered, 'in short, that, thus understood comprises two different during the first flappings, the wings are forms; soaring flight properly so called, like the slats of opent Venetian blinds. for which the bird utilizes the speed The feathers meet the air edgewise. acquired during a course of beating the This arrangement, which offers a mini- wings or by a fall from an elevated point, mum of resistance to the passage through and sailing fight, in which the bird has the air, is caused by an automatic pivot- essential need of the assistance of the ing of the feathers, due to a very com
wind. plicated disposition of their elastic liga- The bird that soars may be compared ments. In proportion as the bird gains to a kite that one draws behind him speed the raising of the wing is caused while running and which rises and by an increasingly weaker action of the keeps itself up if the air is calm. All the middle pectoral muscle, and it becomes rowers of medium size, herons, storks, entirely passive
buzzards, sea gulls when the motion is
and hawks, practice normal. It is in
this kind of flying, deed, the relative
and it is always wind produced by
easy to observe the the speed of the
periods during bird which acts on
which they suspend the convex face of
he flapping of their the wing. Here Fig. 8. APPEARANCE OF A BIRD THAT GLIDES
wings, and continue
RAPIDLY. again we find two The point of the wing is carried backward,
their course, hold
possible from an elevated starting point, it is necessary to fall one yard in order to traverse eight yards. A bird which would soar to the height of 1,000 yards would thus be able to land without fatigue at a distance of 8,000 yards.
Among soaring birds the sustaining qualities commence to gain on the propelling qualities; the concavity of the wing becoming no longer necessary except at the moment of the
flappings, is assured only ing their wings extended almost without by the elasticity of the last quill feathers losing any speed; then a few vigorous opposed to the rigidity of the first; bestrokes of the wings will quickly start sides, the surface of the wing is reduced them again on their way. Such flying is and the tip gains a considerable developa gliding over the air, and the great force ment. Thus there is produced a narrow acquired during the period of the beating wing with a convex edge which is at of the wings is utilized by the bird for once an excellent organ of propulsion taking support on the air and for con- and a very good soarer, qualities indistinuing its course either while rising, pensable for assuring to these birds their remaining at the same height, or de- busy existence in the pursuit of very scending. According as its passage fol- fugutive prey. lows the one or the other of these direc- Clearly then this condition, pushed to tions the speed diminishes rapidly, the extreme, will lead to fight by sailing, slowly, or increases. The bird which practiced only by birds which are no presents most frequently these different longed rowers and which borrow the modes of flying is the falcon in hunting. necessary energy for their movement When from a great height it perceives from another source than their muscles. its prey, it lets itself fall almost vertically Among the large sailing birds only the in such a manner as to return and attack sustaining qualities exist; no more conits victim from beneath; if it fails, it sets cavity of the wing, no more convexity or its wings and its body with the purpose predominance of the tip, but large flat of utilizing the enormous force acquired wings provided with extremely supple during the fall to mount again to a feathers with slight spread of the tip. height when it commences again this The form of the wing itself is modified ; manoeuvre and continues these duckings, the enlargement of the surface is obwithout interruption, and consequently tained not by increasing the span, which without fatigue, until the prey is cap- would be inconvenient for the start, but tured.
by filling up the angle at the end of the The pigeon also offers us frequent ex- wing which from being pointed becomes amples of soaring. When, perched on a rectangular. roof, it wishes to descend to the ground, These are actual observations, and it lets itself fall vertically then, reducing although denied by authors who have its speed by flapping its wings or, if it never witnessed this kind of flying, flight has some space before it, it lets itself by sailing, are today definitely admitted, glide following a parabolic curve which and no longer does any one deny that, places it gently on the ground.
in the large bird, the propelling and susBy observation of different soaring taining powers are obtained, simply by birds and by experiments performed with the reaction of the air in motion, by the soaring machines it has been shown that, wind alone. The agreement ceases when for the bird wishing to land as far as there is a question of explaining the
mechanism of the flight by sailing and in There results in these birds a sort of particular this paradoxical fact that the balancing very clearly noticeable to bird can rise and make headway against observers. This balancing is very much the wind.
diminshed in large sailing birds with Many explanations have been pro- supple wings; the shifting of the wings posed, some fantastic, others the discus- comes into play and as the total mass of sion of which leads to such absurd deduc- the body and of the wings requires much tions as the realization of perpetual greater energy in order to be displaced motion; others again, true perhaps in from its equilibrium, it is the wings alone certain particular cases, could not be that feel the effect of the changes of the embraced in a general theory. To this direction of the wind. And when the last class belong the hypotheses based on variations of the wind are very slight, the utilization of the ascendant currents the feathers alone, the respective indeand on the variations of the velocity of pendence of which makes them like so the wind. It is cer
many small wings tain that the bird
for independent gains in the ascend
shifting, receive the ing currents of the
puff of air and abair; but there have
sorb its energy. been noted also
The large sailing many sailing Aights
birds having to count with the wind hori
only upon the wind zontal or even de
are necessarily conscending ; so that if
as to the theory of the
utilize the lightest ascendant wind suits
breath of air; everyvery well certain
thing with them cases, as also that
tends to this result, which utilizes the
from the sensitivevariations of velocity
ness of the feathers and direction of the
and the suppleness of wind, there remains
the wing, to the abilto be found a gen
ity of spreading the eral theory which
wing like a fan may be applied to all
when, if they wish these different cases. Fig. 10. VULTURE SOARING.
to rise, they have The sailing bird,
need to increase having only the aid of the wind to sup- their sail. This explains the different port it in the air, must be built for positions reproduced in our figures 4 and utilizing the slightest current of air from 8, the first corresponding to a light wind whatever direction it comes.
It is neces- for the utilization of which it is necessary sary also that its organs be delicate to set full sail, the other becoming necesenough to adapt it without delay to the sary when the wind freshens and there changes of the direction of the wind is need of taking a reef. Between these which are almost always very sudden. two extreme cases, there is an infinity These results are assured by the supple- of intermediate situations that the shiftness of the wings themselves and by that ing of the feathers, or that of the wings, of the feathers, since the position of the if it is necessary, suffice to regulate. The plane to the air has to be modified. In quill-feathers, in fact, constitute an autofact each change of direction of the wind matic shifting device which assures the requires, if the wings are not sufficiently longitudinal and lateral equilibrium in supple, an oscillation of the body and of normal conditions; the assistance of the the wings all together. This is what is wing, and of the entire body, are used produced in the semi-sailing birds with only in extremely violent strokes. This the wings relatively rigid, the tips of automatic shifting is indispensable to which are convex and predominant as in birds that practice flight by sailing, and the case of the sea gull and swallow. those which are not provided with it, the