Aves are bipedal, warm blooded vertebrates with an exoskeleton of epidermal feathers and scales. Body temperature is very high and constant in birds. clip_image001Birds resemble reptiles in many characters such as laying of eggs on land and formation of foetal membranes during embryonic development. Due to these common features, reptiles and birds are included in the group Sauropsida Based on the resemblances between the fossil birds and the reptiles, Huxley described birds as the glorified reptiles".

Aves were evolved during Jurassic period of Mesozoic era from ornithischian dinosaurs Birds became modernized in the Cretaceous period. The modernization involved both morphological and physiological changes such as maintenance of constant temperature, fusion of skull bones, formation of pneumatic bones, fusion of vertebrae to form synsacrum. Birds have become highly modified to adapt to an aerial mode of life. There are about 8,590 species of birds which lead aerial, terrestrial and aquatic modes of life.

The study of birds is known as ornithology.


Flying birds are included in the group carinatae. The following are distinctive characters.

1. The fore limbs are modified into wings. Each wing bears three digits and provided with feathers for flight. 

2. The hind limbs are adapted for walking, swimming and perching.
They bear four toes.

3. The body is divided into four regions namely head, neck, trunk and

4. Skin is dry and skin glands are absent, but an oil gland viz., preen gland or uropygeal gland is present at the base of tail in flying birds. Oil gland is meant for preening of feathers.

5. The body is covered by feathers except on beak and legs. These are modified epidermal scales. They form thermal insulation around the body. The scales are present on the legs.

6. The jaws are protruded to form characteristic horny beak or bill. Beak is covered by epidermal sheath called Rhamphotheca. Teeth are absent in the jaws of modern birds.

7. The bones, forming the skeleton, are spongy, light in weight containing air cavities. So the bones are called pneumatic bones. bird-skeleton

8. The skull bones are completely fused. The skull is sutureless and monocondylic.

9.Cervical and free thoracic vertebrae are usually heterocoelous.

10. Last thoracic,lumbar, sacral and anterior five caudal vertebrae are fused to form a synsacrum. synsacrum-birds

11. The posterior caudal vertebrae are usually fused to form a pygostyle (plough shaped bone).


12. The sternum is broad, usually with a longitudinal ventral keel or carinaior the attachment of flight muscles. Ribs are double headed provided with uncinate process.

13. The coracoid is pillar-like, scapula is sabre-shaped.

14. Two clavicles are fused to form into a V-shaped furcula or wish bone or merry thought bone.

15. Distal carpals and metacarpals are fused to form carpo-metacarpus.

16. Proximal row of tarsals are fused with the tibia to form tibio-tarsus. The distal row of tarsals are fused with metatarsals to form tarso-metatarsus.

17. The posterior part of the oesophagus is dialated into crop, and the stomach is divided into proventriculus and muscular gizzard. Cloaca is three chambered. One pair of rectal caecae are found at the junction of the ileum and large intestine.


18. Lungs are spongy and non-elastic and are not suspended in the body cavity. Air sacs are present in lungs and some of them communicate with air cavities of the bones. Alveoli are absent in lungs. bird-lungs-airsacs

19.The voice is produced by the syrinx, situated at the junction of trachea and bronchi. Larynx is without vocal cords and lost the function of sound production.

20. The heart is four chambered with two auricles and two vertricles. sinus venosus is absent. Only the right systemic arch (aortic arch) is present in the adult.

21. Erythrocytes are oval, biconvex and nucleated.

22. Renal portal system is reduced.

23. Kidneys are metanephric and three lobed. Urinary bladder is absent. Nitrogenous excretory waste is eliminated in the form of semi-solid uric acid (uricotelic) bird-urinogenital-system

24. The cerebral hemispheres and cerebellum are large and the optic lobes are displaced laterally. Olfactory organs are poorly developed in birds.

25. Twelve pairs of cranial nerves are present.

26. Eyes are sharp and have pecten. Sclerotic plates are present at the junction of cornea and sclerotic layer.


27. Sexes are separate and sexual dimorphism is well developed.

28. Only left ovary and oviduct are present. Penis is absent in flying birds.

29. Fertilization is internal. All are oviparous.

30. Eggs are megalecithal type containing large amount of yolk. Eggs undergo meroblastic cleavage.

31. Parental care is well developed.

32. Extra embryonic membranes like amnion, allantois, chorion and a large yolk sac are formed during embryonic development.

Ex: Crows, Sparrows, Parrots, Fowls etc.

No comments:

Post a Comment