Introduction to Aves:
The aves or bird are warm-blooded vertebrates particularly adapted for flight. Body is covered with soft feathers that are epidermal outgrowths like scales of reptiles and hairs of mammals. Scales are also there on lower part of legs and on feet. They contain four-chambered heart. Fore limbs are altered to form wing axes (in flying birds) or reduced in non- flying birds. Hind limbs contain clawed digits that are variously altered for perching, walking, running, hopping, or swimming. Head is generally small and rounded, neck is well marked and whole body is oval and streamlined. Tail is short and caudal vertebrae are fused to form the pygostyle. Jaw bones are altered in relation to back and are devoid of teeth.
External Features of Birds:
Body of birds is steamlined or spindle-shaped well adapted for aerial life. Neck is generally long and flexible. Head is rounded and facial portion is produced into beak. Close to base of beak are two slit-like nostrils. Eyes of bird are of significant size for sense of smell appears to be feebly grown but power vision is equally highly developed, particularly in some birds of prey. Every eye is given with three eyelids: upper and lower eyelids are like fold of skin while third eyelid, nictitating membrane, is delicate transparent membrane that can be drawn across eye. Generally lower lid is variable. On every side of head there is a small aperture with the short passage leading to ear-drum. It lies behind and below eyes and is generally hidden by features. Not like mammals, external ears are absent. Fore limbs form wings that are organs of flight while hind limbs are adapted for bearing whole weight of body when walking. For this objective hind limbs are generally joined fairly for forward and skeleton is also altered to this end. Legs are covered with scales. Cloaca lies on ventral surface at root of tail and on dorsal surface of same region is the oilgland. Its oily secretion is utilized for preening feathers. Feet, break and tongue present extremely large number of variations of form that are closely related with habits of birds. Typical number of toes is four, of which, three are directed forwards and one backwards. In perching birds toes are adapted for grasping and automatically clutching support. 3 toes are directed forwards and 1 backwards. Same arrangement of toes is also found in birds of prey that use their feet for seizing. Claws form great talons as in hawks, eagles, kites, falcons, etc. Legs of wading birds are generally very long and partially or entirely unfeathered up to tibial region. They have extremely long toes. Swimming birds such as ducks have webbed feet that serve as paddles.
Adaptive Features of Flight:
i) Wings act as propeller.
ii) Streamlined body is well adapted for movement by air.
iii) There is significant shifting of weight of body from margin to centre due to loss of teeth; furthermore as wings are placed high up on trunk body is prevented from turning over.
iv) Skull is very light and most of bones are welded together.
v) Feathers that clothe a bird, hold blanket of enveloping air next to body; as this air is warmed by body and is as a result lighter than surrounding air, it adds somewhat to buoyancy of bird. Feathers give lightweight wing, tail and body contouring.
vi) Rectum where faeces are carried is very much decreased in length. While flying birds would be greatly handicapped by presence of too much of faecal matter there.
vii) Bones are honeycombed, giving the light but rigid frame.
viii) Keratin beak is lighter than the bony jaw with teeth.
ix) Endothermic metabolism allows highly effective chemical reactions at best temperatures.
x) Feathers give lightweight insulation for protection of body temperature.
xi) Rapid, proficient digestion reduces weight of digesting food.
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