AERIAL ADAPTATIONS IN

BIRDS

Young (1958)- birds as ‘masters of air’

Practically no system, no organ, modified in relation to flight Anatomy, embryology, physiology & ecology modified

1. SHAPE

Represents sums of all its several adaptations

Perfectly streamlined, spindle-shaped body designed to offer minimum resistance in wind

Easily propelled through the air like fish through water

2. Compact body

Compact body, light but strong dorsally & heavier ventrally helps in maintaining balance in air

Attachment of wings high up on thorax, high position of lungs & air-sacs, low & central position of heavy muscles, sternum & digestive organs beneath the midline of attachment of 2 wings & consequently low center of gravity are of importance

3. Body covering of

feathers

Feathers are diagnostic as none other group of animals have them

Smooth, closely fitting & backwardly directed contour feathers make body streamlined & help in passage of air by reducing friction

Light feathers hold a considerable blanket of enveloping air around body & add to buoyancy

Non-conducting covering of feathers insulate body perfectly

Prevents heat loss enabling the bird to endure intense cold at high altitudes & also to maintain a constant temperature

4. Forelimbs modified into

wings

Forelimbs modified into wings

Specially designed structures equipped with special flight muscles as instruments of propulsion through air

Elongated flight feathers (remiges)

Expanded membranous part (vane) of each remex forms a flexible & continuous surface for striking air in flight

Flight feathers form a broad surface for support in air

Particular shape of wing with thick strong edge, convex upper surface & concave lower surface, cause reduction in air pressure above & increase below with minimum turbulence behind

Helping in driving the bird forward & upward during flight

5. Short tail

Short muscular tail bears a series of long, strong but light caudal feathers (retrices)

Arranged in fan-like manner serving as rudder for steering during flight to suddenly check flight & as counterbalance in perching

6. Beak

Conversion of forelimbs into wings is compensated by presence of bill or beak

Mouth drawn into a horny beak, used as a forceps in picking things

Also used for nest building, done by other animals by forelimbs

7. Mobile neck & head

Neck is very long & flexible

Since bill is used for feeding, preening, nest building, offence & defense like mobility of neck & freedom of movement of head are very important

8. Bipedal locomotion

Forelimbs not available, hind limbs spring anteriorly from trunk to balance & support entire weight of body & for locomotion of ground or water

Bipedality is characteristic as flightless birds have all retained the habit of walking on 2 legs

Legs are relatively stronger

10. Large muscles of flight

Muscles of back are greatly reduced, flight muscles on breast highly developed, weighing 1/6th of whole bird

9. Integument

Loose skin is a modification for flight

Responsible for extensive movement of skeletal musculature

Wing is depressed enormously by pectoralis major

Elevated & raised by pectoralis minor, the tendon of which passed through foramen triosseum to be inserted dorsally on head of humerus

Other muscles of minor importance

11. Perching

Hind limbs well suited for arboreal life

Muscles well developed for perching

As bird settles on tree, bending of legs exert a pull on the flexor tendons making toes automatically to flex & to grip the perch

Bird in resting or sleeping is clamped to its perch

12. Endoskeleton

Many adaptations are apparent in skeleton

Fusion of bones with the smallest amount of material after ‘hollow girdle principle’ combines strength with lightness for successful flight

Most bones pneumatic & filled with air-sacs instead bone marrow

Skull bones light & most firmly fused together

Uncinate processes of thoracic ribs help in producing compactness, necessary for flight, by concentrating the mass

Rigidity of dorsal part of vertebral column due to fusion of vertebrae provides a firm fulcrum for action of wings

Heterocoelous vertebrae confer great flexibility & all birds can move their neck through 180° which helps in preening feathers in all parts of the body & food collection

Shortening of caudal vertebrae & formation of pygostyle has assigned stability in air

Fusion of pelvis with synsacrum provides firm attachment to legs, supports the weight of body when bird is walking & counteracts the effect of shock as bird alights

Abundance of mid-ventral symphysis of pubes & ischia results in a more posterior displacement of viscera shifting center of gravity near the hind legs

This permits laying of large eggs with calcareous shells

Fusion of distal tarsals with metatarsus to form tarsometatarsus & proximal tarsals with lower end of tibia to form tibiotarsus help to strengthen the legs for bipedal gait

Strenum is greatly expanded & bears a large mid-ventral ridge or keel for attachment of major flight muscles while it is without keel in running birds

Sternum also supports the abdominal viscera

13. Digestive system

Rate of metabolism is very high, food requirement great & digestion rapid

Food of high caloric value selected, largely utilized with minimum amount of indigestible waste

Consequently digestive system is compact but effective Rectum short because fecal matter is small

14. Air-sacs & respiration

Inelastic lungs of birds are supplemented by remarkable system of air-sacs, which grow out from lungs & occupy all available space between internal organs even extending to the cavities of hollow bones

Air-sacs secure more perfect aeration of lungs & help in internal perspiration, thus, helping in temperature regulation

Avian lungs are completely emptied with each breath there belong no residual air remaining so more effective respiration

While flying movements of wings contribute to respiration by compressing & dilating air-sacs & thus, bird breathes more easily when in flight than at other times

15. Warm-bloodedness

Warm-blooded

Perfect aeration of blood is responsible for high temperature of body (40-46°C) necessary for flight requiring a great output of energy over a longer period

16. Circulatory system

Rapid metabolism & warm-bloodedness require a large oxygen supply & efficient circulatory system

The avian heart is relatively large & completely divided into 4 compartments

Functions very efficiently with a double circulation of blood

High proportion of hemoglobin present in the RBCs of avian blood is also responsible for its quick & perfect aeration

17. Urecotelic excretion

No urinary bladder

Water of excretory fluid is reabsorbed in urinary tubules of kidneys & in coprodeaum of cloaca

Result is formation of a semisolid excreta chiefly containing insoluble uric acid & urates avoided at once

Features help in reducing unnecessary weight of body

18. Brain & sense organs

Birds depend for their main contact with the world upon sight rather than smell

Eyes are large & the large optic lobes correspond to the great development of sight

Eyes occupy a large portion of the head & both eyes together are often heavier than brain

Ability to accommodate rapidly is also well developed for bird must change quickly from a distant to a near vision in flight

Much developed & convoluted cerebellum indicates the delicate sense of equilibrium and great power of muscular coo-ordination belonging to birds

Enormous development of corpus striata in cerebrum also adds to extraordinary actions to attain stability in flight

19. Single ovary

Presence of single functional ovary of the left side in the female also leads to reduction of weight which is so essential for flight

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