Enlist the salient features of class Aves.

The salient features of class Aves: 

1. Habitat: These animals are aerial in habitat. 

2. Locomotion: Forelimbs are modified into wings for flying. Hind limbs are used for walking, clasping tree branches and running. Aquatic birds have webbed toes. This helps in swimming, e.g. Duck.

3. Body division: Body is differentiated into head, neck, trunk and tail.

4. Body shape: Body is streamlined (boat-shaped) to reduce resistance during flight. 

5. Body temperature: These are warm-blooded animals (homeotherms) i.e., keep the body temperature constant irrespective of fluctuations in environmental temperature. 

6. Exoskeleton: Exoskeleton is made up of feathers. Scales are present on hind-limbs. Skin is thin, dry and non-glandular except oil gland at the base of tail (uropygial gland). 

7. Endoskeleton: Bones are hollow (pneumatic) with air cavities to reduce body weight. 

8. Digestion: Jaws are modified into beaks. Teeth are absent. Special structures such as crop and gizzard are present. 

9. Circulatory system: They show double circulation. Blood is red in colour due to presence of biconvex and nucleated RBCs. Heart is perfectly fourchambered, with two auricles and two ventricles. 

10. Respiration: Respiration occurs by lungs. Presence of air sacs increases the buoyancy. 

11. Nervous system: Brain is enlarged with a well developed cerebellum for equilibrium. 

12. Reproduction: Sexes are separate and the animals exhibit prominent sexual dimorphism. 

13. The female shows presence of only left ovary and left oviduct.

 This helps to reduce body weight during flying. Fertilization is internal. Avians are oviparous. Parental care is very well developed.

 e.g. Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Kiwi, Aptenodytes (Penguin), Corvus (Crow), Neophron (Vulture), Passer (Sparrow), Pavo (Peacock), etc.