by Salim Ali ( (1896-1987)
(excerpts from The Book of Indian Birds ( page 317) published by the Bombay Natural History Society www.bnhs.org)
The wing actions of a bird in flight are actually less simple than the up and down flapping they appear to be. They involve a number of complex principles of aerodynamics, which have been more intensively studied and are better understood since the invention and development of the aeroplane and the glider.
Modern aircraft design owes a great debt to the careful analyses of the principles involved in the flight of birds. The improvements continually being made in flying machines keep pace with our knowledge of the mechanics of bird flight.
A remarkable and comparatively recent example of how flying man has profited from the flying bird was the invention of the Handley-Page ‘slotted-wing’ safety device now almost universally used in aeroplanes.
A well-known principle of aerodynamics is that the heavier a plane the faster it must fly to keep itself in the air and prevent ‘stalling’. The use of the slotted wing device enables heavy aircraft to land safely at relatively slow speeds without crashing. It has not only helped to avert innumerable accidents, but also made possible the enormous increase in the size and weight of the planes we see today.
Briefly, by regulating the pressure and angle of the air upon the wings of an aeroplane this contrivance produces what may roughly be described as a parachute or helicopter effect and enables the machine to fly comparatively slowly and land gently. The device is clearly an application of the same feature as is seen in the splayed fingerlike wing - tips of heavy flying birds with relatively broad wings, such as eagles vultures and storks.
If one of the outer primary feathers of a vulture is examined it will be seen that there is a conspicuous narrowing, or ‘emargination’ as it is known, on the outer portion of the vane, i.e. on he outer web. The gaps or slots between the outspread primaries (or ‘fingers’) of a soaring vulture are formed not only by the normal spreading of the primaries but they can be further considerably widened by these emarginations. A clever friction surface on the web prevents the feathers from splaying beyond a certain limit.
By manipulating the slots between the primaries, so as to offer varying and controlled resistance to the air, rather in the manner of venetian blinds or shutters, the heavy bird is enabled among other advantages to soar in comparatively still air. It also permits accurate landing at a speed, which would ordinarily be unable to withstand the pull of gravity, and bring the bird crashing to earth, unless it steadied itself by flapping.
The bird’s tail is an important accessory of flight and assumes a variety of shapes and sizes, e.g. the short rounded tail of the eagle and the vulture, the deeply forked tail of the swallow, the tern and the kite, and the long graduated tail of the tree pie. Its primary function is that of balancing and steering. If accidentally lost, the bird’s movements in the air are considerably hampered; but it is not rendered completely helpless since much of the steering, balancing and braking is also done by the wings. The tail further helps in regulating height in the air, and of course serves as a brake in checking momentum. It is noticeable that the alighting or landing of a bird is invariably preceded by an upward tilt of the long axis of its body accompanied by the vigorous flapping of wings and a depressing and expanding of the tail.
No better example can be given of the perfect muscular control over wings and tail and the coordinated efficiency of the various special adaptations of the flying bird (viz. Streamlining, flexibility of neck, sideways position and rapid focusing of the eye, etc.) than the Alpine Swift. Cruising at speeds which may be anything up to 250 km p.h., this bird is capable of shooting directly and with effortless grace up into the narrow fissure a rock cliff that holds its nest.
The ease and skill with which the parent Baya, flies into the narrow entrance tube of its swaying pendant nest, with no hesitation or slackening of speed;. The stooping of a Pariah Kite upon a dead rat lying in the middle of a congested city bazaar, with its dexterous turns and twists to avoid the traffic below and the tangle of telephone wires overhead --- are sights familiar to every bird watcher, and such as never cease to thrill.
It is useful to be able to recognise the style of flight adopted by various groups and species of birds and their sundry little mannerisms on the wing since these are points that may assist materially in field identification. A familiarity with the wing action and outline of a bird high up in the heavens, or when silhouetted against the darkening sky……… or in the mid afternoon glare…… is particularly helpful in observations relating to migrating birds. The three main types of flight to be differentiated are:
1. THE normal flapping flight which may be roughly compared to swimming breast-stroke. In this the wings move upward and forward, downward and back (under the plane of the body) and then upward again, in a sort of rotary motion. The upstroke is very rapid but owing to the curvature on the wing surface (the camber), the partial folding of the wing and the set of the feathers, a minimum amount of resistance is created. The free movement of the wing makes this upstroke resemble the ‘feathering’ of an oar when rowing. The down stroke (or power stroke) is made with the wing fully outstretched exerting its maximum push on the air. It helps both to lift the bird and propel it forward. The crow and most of the Passerine or Perching birds may be cited as examples of flapping fliers. The wing-beats vary according to size of bird and speed of flight: while the sparrow has about 13 per second, the pelican has only 1 to 1.5! A short, rounded wing (as, for example, in the Jungle Babbler or Spotted Owlet is the mark of a weak flier or a more or less sedentary bird. Long, pointed wings indicate strong sustained flight often against heavy head-wind, and are possessed by pigeons, falcons, swifts and all birds that undertake long migratory journeys, e.g. sandpipers, wagtails, etc
2. Gliding involves sailing on outstretched motionless wings taking advantage of wind currents. It may be compared to ‘free-wheeling’ or coasting downhill on a bicycle. Unless launched from a cliff or other elevated position the bird requires a certain amount of initial flapping to produce the required momentum. And if the momentum is not renewed from time to time by further wing-beats, loss of height results. Gliding is best seen in gulls circling effortlessly round a ship looking for scraps thrown overboard. Typical gliding birds have rather narrow and long tapering wings without outspread ‘fingers’ at the tip.
3. Soaring is perhaps the most spectacular style of bird flight. It differs, little from gliding except that ascending air currents, or thermals, play a predominant part in it. By various manipulations of its outstretched wings and tail, the soaring bird takes advantage of every current and eddy to gain height. The upward soaring of vultures and storks is achieved in loose, wide spirals like the coils of a spring. As the bird glides downwind it gains in momentum but loses somewhat in height. By a slight tilt of the body axis as it wheels round to face upwind, the bird is lifted upward without effort, and gradually gains height, spiral upon spiral, until it soon becomes a speck in the heavens. Since soaring depends largely upon ascending currents it is seen at its best in warm regions of the globe such as ours, and here only between the hours of sunrise and sunset when the heated air rushes up to higher levels. Birds that habitually soar, such as vultures, eagles, storks, pelicans, etc., have relatively broad wings with rounded or squared wing – tips terminating in a finger – like spread of the outer primaries – the ‘slotted-wing device’.
Besides these three main types of flight one must learn to recognise their variations and combinations. For instance woodpecker or wagtail flies by a succession of rapid wing-beats followed by a short pause in which the wings are pressed to the sides of the body. This results in a forward dip and a slight loss of height and produces the well-known undulating effect so characteristic of these species. In other birds, e.g. partridge, shikra, brainfever bird, rosy starling, etc., a succession of rapid wing-beats is followed by a short glide on outstretched motionless wings - - free wheeling – in which the bird does not loose height; its flight is direct and not undulating.
The hovering flight of certain birds needs special mention. Its foremost exponents among our Indian species are certainly the Kestrel and the Pied Kingfisher, but Harriers the Blackwinged Kite, fishing eagles and some other birds also frequently indulge in it. Hovering enables the bird to poise itself stationary in mid-air and survey the ground or water below for its prey. It is really a variant of the normal flapping flight but always attained head-to-wind and with an upward tilt of the body axis so as to offer maximum resistance to the lateral current of air. The wing or wing-tips are flapped rapidly to ‘tread’ the air as it were, and the bird thus remains suspended for many seconds at a time. Hovering flight on rapidly vibrating wings is also seen in the sunbirds as they probe into flower tubes for nectar or in search for insects poised in front of a flower. It has reached its perfection amongst the humming-birds. In some of these scarcely larger than a bumble-bee, the wing strokes have been ascertained to be between 20 to 50 per second. (1200-3000 per minute).
Whether it be the effortless, leisurely soaring of the vultures in the firmament or the swishing lightning stoop of the falcon upon its quarry, the loud whirring flush of the startled partridge in the corn-field or the silent ghostly glide of the questing owl in the gloaming, it is all the manifestation of the same remarkable process of evolution that has culminated in the flying bird, raising it as if by magic from the lowly cold-blooded reptile to this bundle of superabundant energy – a graceful and buoyant creature with a mastery of the air that Man, with all his ingenuity and cunning, is never likely seriously to challenge.
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