Lecture notes on flight adaptations in birds

Morpholgical,anatomical and physiological changes or developments in the organization of an animalwhich are suited to a particular environment is called adaptation.Birds are the masters of air whose bodies are highly specialized for aerial mode of life. In aerial birds every part of their body has been modified for aerial life. To fly in air, they have to possess following essential specializations in their morphology, anatomy and physiology

A. Morphological adaptations

1. Body shape : Because speed is a must for aerial life, so, to minimise the resistance offered by air during fight, the body of most birds is fusiform or spindle-shaped and it lacks any extra projection which may offer resistance in the attainment of speed in air.

2. Body-covering of feathers :Body is  covered by special skin derivatives called feathers

(a).The smooth closely fitting and backwardly directed contour feathers make the body stream-lined and reduces the friction

(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 buòyancy.

(d) The non-conducting covering of feathers insulates the body perfectly and prevents loss of heat to maintain a constant temperature.

(e) Feathers of wings form a broad surface for striking the air.

3. Organs for flight:The forelimbs have transformed into unique and powerful propelling organs, the wings. The wings are the sole organs of flight.

4. Mobile neck and beak : The transformation of forelimbs into wings is duly compensated by the extreme mobility of the long and flexible neck for reaching food. 

5. 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.

6. Short tail : The short tail of a bird bears a tuft of longtail feathers or rectrices, which spread out in a fan-like manner and serve as a rudder during flight. They also assist in steering, lifting and counter-balancing during flying and perching.

B. Anatomical Volant Adaptations

Flying birds have following anatomical modifications for volant life :

1. Flight muscles: 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.

2.Skeletal system

a. Preumatic bones- The bones are light, hollow and provide more space for muscles attachment. Due to pneumatic nature weight of the body is reduced which is helpful in flight.

b. Skull- Bones of skull are fused and light; teeth are absent in jaws. This reduce the weight of the body.

c. Fused thoracic vertebrae- The first four thoracic vertebrae are fused to form a firm fulcrum for working of the wing. In flightless birds these are free.

d.Synsacrum- Last thoracic, lumber sacral and caudal vertebrae fused to form synsacrum. Pelvic girdle & synsacrum provide plate like out come to support the weight of the body.

e. Sternum-Sternum is well developed and support the viscera. It also provide the surface for the attachment of flight muscles.

f. Pectoral girdle-It is strong. The coracoid is stout pillar like to resist the down stroke of the entire wing.The clavicle and interclavicles fused to forms v-shaped bone called, furcula which help in flying.

g. Pygostyle-The last four caudal vertebrae are fused to form the pygostyle which offer a strong attachment

to the rectrices.

h. Ribs with uncinate processes

3. Digestive system : The rate of metabolism in birds is very high, food requirements are great and digestion is rapid. Most birds are very selective in their diet and accordingly their beaks

are variously modified. Further, bécause undigested waste is minimum and is immediately got rid off, 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 to reduce weight. 

4. Respiratory system : As a flying bird requires great and sustained power, therefore, its respiratory system remains specialized to meet the extensive rate of metabolism greater amount of oxygen molecules is needed by the body tissues. For this purpose, the dense,

inelastic and complicated lungs are supplemented by a remarkable

system of air sacs,, which grow out from lungs and occupy allavailable 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 is called double respiration.  The air sacs help in regulating body temperature by internal perspiration. 

5. Circulatory system : Rapid metabolism requires large oxygen supply to the tissues, which can be achieved by an efficient cireulatory system. powerful and efficient. Due 'to double circulation in it, the oxygenated and deoxygenated bloods remain completely separated.

Further, red blood cells of birds contains large amount of haemoglobin and this is responsible for blood's quick and perfect aeration.

6. Warm-bloodedness : In birds the body temperature remains high (40°-46°C) and does not change with the change of environmental temperature.  The high and constant body temperature enables the bird to take flights at high altitudes and also facilitates activeness in every season.

7. Excretory systems: The avian excretory system becomes specialized in three ways : 1. For the retention of water, the uriniferous tubules of avian metanephric kidneys are added with Henle's loops, which are efficient in water absorption. coprodaeum of cloaca is another efficient water absorbing organ of birds. 2. 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. 3. 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.

8. Brain and sense organs: The avian brain is highly developed consisting of well-developed centres of equilibrium, muscular coordination and instinct. Hence, the cerebellum and cerebrum are highly developed.  Because birds have to depend mostly on the sense of sight, so, the eyes and optic lobes of brain are well developed.Presence of pecten in the eye for assisting vision. 

9. Reproductive organs: In female birds the ovary and oviduct of one side (i.e., right side) of body are preserved. This necessarily reduces the weight of body.