- Morphological Adaptations
- Anatomical Adaptations
Most birds possess following important flight or volant adaptations
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.
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.
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.
Feathers have following advantages for birds
- 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.
- The feathery covering makes the body light and at the same time protects from the hazards of environmental temperature.
- The feathers hold a considerable blanket of enveloping air around the body and add much to its buoyancy.
- 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.
- Feathers of wings form a broad surface for striking the air.
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 surface area of the wings is increased by the development of elongated flightfeathers, 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.
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.
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 hindlimbs occupy a somewhat anterior position on the trunk and become stouter in case of ratites which are running birds
The hindlimbs 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.
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.
Flying birds have following anatomical modifications for volant life:
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.
The muscle fibres comprising the flight muscles are of striated type and well vascularised to withstand fatigueless after prolonged activity. The wings are depressed by large muscles, pectoralis major and elevated by pectoralis minor.
Other muscles are small and help the above muscles in their functioning.
Lightness and Rigidity of Endoskeleton
The skeletal framework of flying birds is very stout and is lightly built on the “hollow-girder principle”. Most of the bones are pneumatics, filled with air sacs and provided with a secondary plastering to make them rigid. Bone marrow is lacking in bones of birds. Further skeletal framework becomes compact, centralised, rigid due to fusion of bones.
Endoskeleton of birds
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.
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.
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.
The shortening of caudal vertebrae and formation of pygostyle has assisted stability in air.
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.
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.
- 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.
- 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.
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.
As a flying bird requires great and sustained power, therefore, its respiratory system is specialised in such a fashion that the food is combusted (oxidised) rapidly and completely to liberate large amount of energy.
To meet the extensive rate of metabolism greater amount of oxygen molecules is needed by the body tissues.
For this purpose, the dense, compact, and inelastic lungs are supplemented by a remarkable system of air sacs, which grow out from lungs and occupy all available space between internal organs, even extending to the cavities of hollow bones.
The air sacs primarily reduce the specific gravity of the bird and also facilitate complete aeration of the lungs.
The avian lungs are aerated twice at each breath which secures perfect oxygenation of blood. The air sacs help in regulating body temperature by internal perspiration.
Further, insertion of air sacs in between the flight muscles like pads reduces mechanical friction and increases the mobility in muscular action.
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.
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.
The avian excretory system becomes specialised in three ways
- For the retention of water, the uriniferous tubules with Henle’s loops are efficient in water absorption. The coprodaeum of cloaca is another efficient waterabsorbing organ of birds.
- For reducing the weight of body, there occurs no urinary bladder and the semisolid urine is immediately excreted out, not retained for long in the body.
- 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.
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.
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. Thus, it becomes evident that birds are fully developed for terrestrial, arboreal and aerial environments.