Flight adaptations in birds

all those interested in birds

1. Flight Adaptations in Birds Dr. Gauri Haval Abasaheb Garware College Pune

2. A BIRD has been described as ‘Feathered Biped’ A bird flies on the principle of an aeroplane, or heavier-than-air machine, rather than on that of a balloon or lighter-than-air machine. The flight is a highly spontaneous action. Larger birds either run or swim rapidly together enough forward momentum for a take off. Smaller birds usually take a quick jump by means of their legs followed by the beating of their wings.

3. A perfect flight is certainly the birds most outstanding qualification. Two main types of flight or aerial adaptation of birds are: 1. Morphological Adaptations 2 . Anatomical Adaptations. 

4. Morphological Adaptations : i . Body Contour : Because speed is a must for aerial life, so, to minimise the resistance offered by air during flight, the body of birds is fusiform or spindle-shaped and it lacks any extra projection which may offer resistance in the attainment of speed in air like fish in the water.

5. ii. Compact Body: Their compact body is light and strong dorsally and heavy ventrally which helps in maintaining equilibrium in the air. The attachment of wings high upon the thorax, the high position of light organs like lungs and sacs, and low central position of heavy muscles, sternum and digestive organs below the attachment of both the wings and consequently low centre of gravity are other morphological facts of great significance.

6. Body-Covering of Feathers: Body of all birds is covered by special integumentary derivatives called feathers. Feathers are diagnostic of birds, since no other group of animal kingdom has ever developed them . Flight feathers, in particular, are masterpieces of propulsion and adaptation, helping penguins swim, eagles soar and hummingbirds hover. Despite such diversity, the  feather  shares a common core design: a one-style-fits-all model with option trims for specialized performance.

7. Feathers have following advantages for birds: (a) The smooth, closely fitting and backwardly directed contour feathers make the body streamlined helping them to pass through the air by reducing the friction to the minimum. (b) The feathery covering makes the body light and at the same time protects from the hazards of environmental temperature . (c) The feathers hold a considerable blanket of enveloping air around the body and add much to its buoyancy. (d) The non-conducting covering of feathers insulates the body perfectly and prevents loss of heat which enables the bird to endure intense cold at high altitudes and also to maintain a constant temperature. (e) Feathers of wings form a broad surface for striking the air.

8. iv. Forelimbs Modified into Wings: The forelimbs have transformed into unique and powerful propelling organs, the wings. The wings are the sole organs of flight. These organs have complicated structural constructions consisting of a framework of bones, muscles, nerves, blood vessels, feathers, etc. Both wings spring from the anterior region of trunk. During rest they remain folded against the sides of the body, but during flight they become expanded . The wing combines strength with lightness and flexibility. The motive power operating the wings is supplied by strongly developed pectoral muscles. The sternum is provided with a deep keel or carina to the edge of which most of these muscles are attached.

9. The surface area of the wings is increased by the development of elongated flight-feathers, the remiges . The vane of each remixes forms a flexible and continuous surface for striking the air in flight. The flight feathers of a wing also form a broad surface for supporting the bird in air. The particular shape of the wing, with thick strong leading edge, convex upper surface and concave lower surface, causes reduction in air pressure above and increase below, with minimum turbulence behind. This helps in driving the bird forwards and upwards during flight.

10. v. Mobile Neck and Head : The transformation of forelimbs into wings is duly compensated by the presence of beak or bill used for feeding, nest building, preening, and offence and defence. The mouth is drawn out into a horny beak which acts as a pair of forceps in picking up the things and in various other activities such as nest building, pruning, etc., which are normally done by forelimbs in other animals. The neck in birds is also very long and flexible for the movement of head necessary for various functions.

11. vi. Bipedal Locomotion: As anterior part of the body of birds becomes concerned with flight, the posterior part of body becomes modified for movement on land. For locomotion on the ground and to support the entire body weight, the hind limbs occupy a somewhat anterior position on the trunk and become stouter in case of ratites which are running birds.

12. vii. Perching: The hind limbs of a bird are well specialised for an arboreal life. Their muscles are developed in such a manner that when a bird sits on a branch of the tree, the toes close round the twig automatically. This happens due to so called perching mechanism. When the bird settles on the branch of a tree, the legs are bent and put the flexor tendons on the stretch. With the exertion of the pull, the toes are bent spontaneously around the perch. A bird can go to sleep in this position without any fear of falling off.

13. viii. Short Tail : Tail is an important accessory of flight and assumes a variety of shapes and sizes For example short rounded tail of Eagle , deeply forked tail of swallow, long tapering tail of tree pie .

14. The short tail of a bird bears a tuft of long tail feathers or rectrices , which spread out in a fan-like manner and serves as a rudder during flight. They also assist in steering, lifting and counterbalancing during flying and perching. The tail further helps in regulating height in the air, serves as a brake in checking momentum. The landing of the bird is preceded by an upward tilt of long axis of body accompanied by vigorous flapping of wings and depressing and expanding of tail.

15. The various requirements of flight are met with by birds. 1. Centralization . The internal structures are well centralized, with certain structures reduced and others eliminated.

16. Anatomical Adaptations Flight Muscles The flight muscles of most birds are red in color ("dark meat") because of the presence of many fibers containing red oxygen-carrying compounds, myoglobin and cytochrome. They are also richly supplied with blood and are designed for sustained flight. Lighter-colored muscles ("white meat"), with many fewer such fibers, are found in pheasants, grouse, quail, and other gallinaceous birds. These are also well supplied with blood, are apparently capable of carrying a heavy work load for a short time, but fatigue more rapidly. If a quail is flushed a few times in a row, it will become so exhausted it will be incapable of further flight. The action of the wings is controlled by the flight muscles which are greatly developed, weighing about one-sixth of the entire bird, while the muscles of the back remain greatly reduced.

17. ii. Lightness and Rigidity of Endoskeleton: One of the requirements of heavier-than-air flying machines, birds included, is a structure that combines strength and light weight. One way this is accomplished in birds is by the fusion and elimination of some bones and the " pneumatization " (hollowing) of the remaining ones. Some of the vertebrae and some bones of the pelvic girdle of birds are fused into a single structure, as are some finger and leg bones -- all of which are separate in most vertebrates. And many tail, finger, and leg bones are missing altogether. Not only are some bones of birds, unlike ours, hollow, but many of the hollows are connected to the respiratory system. To keep the cylindrical walls of a bird's major wing bones from buckling, the bones have internal strut-like reinforcements . Evolution has created in the avian skeleton a model of parsimony, lightening where possible, adding weight and strength where required. The results can be quite spectacular: the skeleton of a frigatebird with a seven-foot wingspan weighs less than the feathers covering it!

18. Endoskeleton of birds thus contains the following characteristics : The skull bones are paper-like thin and show a tendency towards the reduction in their number. These bones are firmly fused with each other. The posterior portion of the skull is spongy. Teeth are lacking. (ii) All the thoracic vertebrae except the last are fused into a single mass giving rigidity to the dorsal part of vertebral column. Fusion of vertebrae provides a firm fulcrum for the action of wings in striking air. The uncinate processes of thoracic ribs help in providing compactness, necessary for flight by concentrating the mass. The arched clavicles fused with interclavicle and powerful pillar-like coracoids of pectoral girdle are well suited to resist the inward pressure of the down-stroke. (iii) The heterocoelous vertebrae confer great flexibility and birds can move their neck through 180°, which help in preening feathers in all parts of the body.

19. (iv) The shortening of caudal vertebrae and formation of pygostyle has assisted stability in air. (v)   Sternum or breast bone is expanded having a median ridge or keel for the attachment of major flight muscles in flying birds, while it is without a keel in running birds, like ostrich. Sternum is also hollow in which viscera are located. (vi)   The fusion of the pelvis with synsacram (viz., fused mass of last thoracic vertebra + lumbar vertebrae + sacral vertebrae + few anterior caudal vertebrae) not only supports the weight of the body when the bird is walking, but also counteracts the effect of shocks as the bird alights. The absence of a ventral symphysis of ischia and pubis permits laying of large eggs . (vii)   The fusion of distal tarsals with the metatarsals to form a tarsometatarsus , and that of proximal tarsals with the lower end of tibia to form a tibiotarsus , help to strengthen the legs for bipedal gait. (viii)   The skeleton of forelimbs is completely modified for the attachment of feathers ( remiges ) and flight muscles. There are only three digits, which are more or less fused.

20. Digestive System: The rate of metabolism in birds is very high, so the food requirements are great and digestion is rapid. Most birds are very selective in their diet and accordingly their beaks are variously modified. Further, because undigested waste is minimum and is immediately got rid of, consequently the rectum becomes much reduced in length and never stores the undigested food. The ill-development of rectum of flying birds indicates towards the fact that the flying animals cannot afford to bear the weight of faeces . The absence of gall bladder in birds minimises the bodyweight to some extent.

21. Respiratory system The respiratory system of birds is also adapted to the demands of flight. A bird's respiratory system is proportionately larger and much more efficient than ours -- as might be expected, since flight is a more demanding activity than walking or running. An average bird devotes about one-fifth of its body volume to its respiratory system, an average mammal only about one-twentieth. Mammalian respiratory systems consist of lungs that are blind sacs and of tubes that connect them to the nose and mouth. During each breath, only some of the air contained in the lungs is exchanged, since the lungs do not collapse completely with each exhalation, and some "dead air" then remains in them. In contrast, the lungs of birds are less flexible, and relatively small, but they are interconnected with a system of large, thin-walled air sacs in the front (anterior) and back (posterior) portions of the body. These, in turn, are connected with the air spaces in the bones. Evolution has created an ingenious system that passes the air in a one-way, two-stage flow through the bird's lungs. A breath of inhaled air passes first into the posterior air sacs and then, on exhalation, into the lungs. When a second breath is inhaled into the posterior sacs, the air from the first breath moves from shrinking lungs into the anterior air sacs. When the second exhalation occurs, the air from the first breath moves from the anterior air sacs and out of the bird, while the second breath moves into the lungs. The air thus moves in one direction through the lungs. All birds have this one-way flow system; most have a second two-way flow system which may make up as much as 20 percent of the lung volume.

22. Circulatory System: Rapid metabolism requires large oxygen supply to the tissues, which can be achieved by an efficient circulatory system. Accordingly, avian heart is large- sized, four-chambered, powerful and efficient. Due to double circulation in it, the oxygenated and deoxygenated bloods remain completely separated. Further, red blood cells of birds contain large amount of haemoglobin which is responsible for quick and perfect aeration of body tissues . A bird's heart is large, powerful, and of the same basic design as that of a mammal. It is a four-chambered structure of two pumps operating side by side. One two-chambered pump receives oxygen-rich blood from the lungs and pumps it out to the waiting tissues. The other pump receives oxygen-poor blood from the tissues and pumps it into the lungs. This segregation of the two kinds of blood (which does not occur completely in reptiles, amphibians, and fishes) makes a bird's circulatory system, like its respiratory system, well equipped to handle the rigors of flight.

23. Warm-Bloodedness: Due to perfect aeration of blood, the body temperature remains high (40°- 46°C) and does not change with change of environmental temperature. For this reason birds are called warm-blooded or homeothermal animals. The high and constant body temperature enables the bird to take flights at high altitudes and also facilitates activeness in every season.

24. Excretory System: The avian excretory system becomes specialised in three ways: ( i ) For the retention of water, the uriniferous tubules with Henle’s loops are efficient in water absorption. The coprodaeum of cloaca is another efficient water-absorbing organ of birds. (ii) For reducing the weight of body, there occurs no urinary bladder and the semi-solid urine is immediately excreted out, not retained for long in the body. (iii) The metabolic nitrogenous wastes are converted into less toxic and insoluble organic compounds such as uric acid and urates , which is an important physiological volant adaptation.

25. Brain and Sense Organs: The avian brain is highly developed consisting of well-developed centres of equilibrium, muscular coordination and instinct. The cerebellum is much developed and convoluted controlling the sense of equilibrium and muscular coordination. The cerebrum is also large and relatively smooth controlling voluntary movements, behaviour , intelligence and memory. Birds have to depend mostly on the sense of sight, so, the eyes are large and optic lobes are well developed due to acute vision. Sense of smell is poorly developed corresponding the ill-development of olfactory lobes.

26. Reproductive Organs: In female birds, the presence of a single functional ovary of left side also leads to reduction of body weight which is essential for flight. But in the case of birds of prey generally both ovaries and oviducts are present. This is because during hunting these birds have to pounce on the prey with great force and struggling prey can kick and break the eggs in reproductive system. Thus, it becomes evident that birds are fully developed for terrestrial, arboreal and aerial environments.

27. The evolution of flight has provided birds with many physical features in addition to wings and feathers. One way to reduce weight in birds is by the fusion and elimination of some unnecessary bones and the “ pneumatization ” of the remaining ones. Not only are some bones of birds hollow but many of the larger ones are connected to the air sacs of the respiratory system. To keep the cylindrical walls of a bird’s major wing bones from sudden change the bones have internal compressive framework. Fusion of bones in birds makes the skeleton light as well as strong. Coracoid, furcula , and scapula form a strong and well built tripod for supporting the wings and broad surfaces for the attachment of large flight muscles. One key adaptation is the fusing of caudal bones into single pygostyle which supports the tail feathers. Birds also lack teeth or even a true jaw, instead having evolved a beak, which is more lightweight. Birds have uncinate processes on the ribs. These are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them. There is practically every organ and system has been modified in relation to flight.

28. References Vertebrates by Kotpal Notes on Zoology The Book of Indian Birds by Dr. Salim Ali