FLIGHT AND FLIGHT ADAPTATIONS IN BIRDS
Flight and Flight Adaptation in Birds
I. Mechanism of Flight.
Three distinct types of flight may be considered gliding, soaring and flapping.1. Gliding. This is the simplest type of flight. In it a bird, after attaining a certain velocity or after reaching a certain height, planes through the air without moving the wings. This type is also used for landing. In gliding (Fig. 7.32A), the wing is moved against the air with its strong leading (front) edge tilted upward. Air passing over the convex upper surface of the wing encounters less resistance and, therefore, speeds up and tends to pull away. This creates above the wing a drop in pressure (semivacuum), producing a 'suction zone' there.
The air flowing over the lower concave, temporarily impermeable surface of the wing encounters greater resistance and is retarded. This causes under the wing a rise in pressure, producing an upward thrust.
These two forces, suction above and upward thrust below, cause lift, the force which keeps the bird aloft. Increase in the tilt of the leading edge of the wing results in an increase in the lift force. As long as the air flows smoothly, the wing remains aloft. But if it is tilted sharply, turbulence sets in above the wing (Fig. 7.32C), the lift is destroyed, and the bird begins to stall.
The air also tends to push the wing horizontally backward. Thus, the total force of the wind on the wing can be resolved into "lift" and 'drag' components (Fig. 7.32B).
A gliding bird with motionless wings gradually falls to the ground in still air. In moving air, however, both the lift and the drag forces are increased and the bird glides horizontally, if the air rises at the same velocity as the bird loses height.
2. Soaring. Soaring on spread, motionless wings is achieved by minor, invisible adjustments in wing outline or position, making use of rising currents rocks provides upward "thermals" in which the of air. Warm air rising from the sun-heated soil or rocks provides upward "thermals" in which the bird circles to maintain its position. Soaring is possible on a large scale in birds with a large wing span e.g., vultures, eagles, albatross, etc.
3. Flapping Flight. Flapping flight is more useful than gliding. Its mechanism is very complicated and varies in its details in different birds. In brief, it involves an effective downstroke and a recovery upstroke. In downstroke, the wings are fully spread and their primary feathers are kept firmly overlapping each other to present a closed, impermeable surface to the air. The downstroke is vertical to begin with and this gives a powerful lift. Then the wings are moved forward with their front edges tilted upward and this gives lift as in gliding. The upstroke is upward and backward and this gives a forward push. For the upstroke, the wings are partly folded and their primary feathers are opened to allow air to slip through, making it easier to lift the wings. Pigeon can beat is wings eight times per second.
Steering. Steering is effected by the tail and by unequal stroke of the wings on the two sides.
II. Muscles of Flight
(Fig. 7.33). The muscles of the breast and forlimbs (wings) are highly developed to play a role in flight. They are called flight muscles.The up and down movements of the wing in flight are brought about mainly by two muscles, namely, the pectoralis major and the pectoralis minor (supracoracoideus subclavius).
1. Pectoralis Major. The pectoralis major is a very large muscle, having about one fifth of the total body weight. It is red in colour, being highly vascularized. Its fibres arise from the whole of the keel of the sternum and from the clavicle, and converge for insertion over a small area on the ventral side of the humerus near its head. Contraction of this muscle pulls the humerus downward, causing a powerful downstroke of the wing. Hence, the muscle is also called the depressor muscle. Downstroke of the wings lifts the bird's body in flight.
2. Pectoralis Minor. The pectoralis minor is a much smaller muscle. Its fibres arise from the anterior part of the body of the sternum, dorsal to the pectoralis major, and converge to a strong tendon. The latter passes through the foramen triosscum (an aperture between the clavicle, scapula and coracoid) and is inserted on the dorsal side of the humerus, again near its head. The foramen acts as a pulley, changing the direction of the action of the muscle. With the result, the contraction of this muscle pulls the tendon, which lifts the wing, casing an upstroke of the wing. The pectoralis minor is, therefore, also called the elevator muscle.
Two small muscles, named coracobrachilalis longus and coracobra-chialis brevis, assist the pectoralis major muscle in the downstroke of the wing. These muscles are, therefore, called the accessory depressors. They extend from the coracoid to the postaxial side of the head of the humerus above the pectoralis muscles.
The wing is rotated in the glenoid cavity by the relatively small scapulo-humeral and coracohumeral muscles. These muscles extend from the scapula and coracoid to the humerus.
The patagia of the wing are kept tensely stretched during flight by tensor patagii muscles present in the patagia. The prepatagium has three such muscles tensor longus, tensor brevis and tensor accessorius. The postpatagium has a single such muscle, viz., tensor membrane posterioris alae.
The intrinsic muscles of the arm also play an important role in flight. The extensor carpiradialis and the extensor carpiulnaris of the forearm aid inradialis and the extensor carpiulnaris of the forearm aid inulnaris of the forearm aid in stretching and folding the wing in cooperation with the upperarm muscles called biceps and triceps. The extensor muscles extend from the radius and ulna to the carpals. The biceps and triceps arise from the scapula and are inserted on to the radius and ulna respectively.
The muscles of the digits, though reduced, alter the of disposition of individual parts the wing and even of individual feathers during flight.
III. Adaptations for Flight. (IMPORTANT)
Adaptations for flight are found practically in all the systems of a bird. The more obvious of these are dicussed below:1. Streamlined Body. Birds have a boat-shaped body, which offers minimum resistance to air during flight. All contour feathers lie flat and are directed backward for the same purpose.
2. Flight Organs. The flight organs of a bird are a pair of large wings, which are modified forelimbs beset with special firm feathers, the long quills and the small coverts. The wings have thick, strong leading edges; thin, flexible trailing edges; convex upper surfaces and concave lower surfaces. This form of the wings efficiently parts the air in front and causes a minimum turbulence behind.
3. Flight Muscles. Massive muscles are present to work the wings during flight. These include the pectoralis major, which brings about a powerful downstroke, and the pectoralis minor, that effects an upstroke.
4. Skeletal Surface for Flight Muscles. Large sternum produced into a large midventral keel provides sufficient hard surface for the attachment of the massive flight muscles.
5. Energy for Flight. Respiratory and circulatory systems efficiently meet the energy requirement of the flight muscles. Blood is aerated during both inspiration and expiration. Unusually large, fast-beating, four-chambered heart supplies oxygenated blood to the wing muscles as well as to the rest of the body by way of a single aortic arch. Another aortic arch, if present, would have mixed the two samples of blood and this would have reduced the metabolic rate.
6. Lightness. Lightness is essential for flight. It has been achieved by birds in many ways. Paper-thin skull bones; pneumaticity of all long bones and many skull bones; complete loss of some bones, two fingers, one toe, claws of fingers and all teeth; reduced tail; short rectum; lack of urinary bladder and loss of one ovary are some of the measures to reduce weight of the body. Pigeon is hardly 500 gm. in weight, and a greater part of it is contributed by the flight muscles.
7. Rigidity. Rigid body can successfully withstand the pressure of the air currents during flight. Rigidity has been acquired in birds by fusion and flattening of bones without affecting lightness in any way. All vertebrae, except the cervicals, are fused. Cranium is almost without sutures. Thoracic compressed vertebrae are to form a firm fulcrum for wing strokes. Double-headed ribs, uncinate processes and sternum form a strong thoracic basket. Fusion of the clavicles adds to the strength of the pectoral girdle. and sternum form a strong thoracic basket. Fusion of the clavicles adds to the strength of the pectoral girdle.
8. Equilibrium. Equilibrium is another essential requirement for flight in the air. Attachment of wings high up on the trunk ; high position of light organs, such as the lungs; low position of heavy flight muscles and digestive organs lower the centre of gravity and prevent turning over of the body in the air.
9. Sense Organs. Olfactory organs have become reduced in view of the fact that the birds flying high in the air cannot depend on smell. The organs of sight and hearing are, on the contrary, of high order, being capable to guide the bird from a distance.
10. Well Developed Nervous System. Birds have a well developed nervous system to coordinate the complex movements of wings and their feathers during flight, particularly of the flapping type.
11. Steering Device. Tail feathers assist in steering, so essential during flight.
12. High Temperature. Birds are homoiothermous vertebrates and maintain a very high temperature with the aid of brisk metabolism and an insulating coat of feathers. Only at a high temperature is strenuous activity, like prolonged flight, possible.
13. Take Off. The legs strengthened by the fusion of their bones are not only effective in dipedal locomotion but also provide a forceful take off for flight.
Storage of food in the crop and water conservation by reabsorption in long Henle's loops and urodaeum enable the birds to undertake long flights, prolonging the need for food and water.
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