Write short note on class - Aves. 

Explanation

The characteristic features of Aves (birds) are the presence of feathers and most of them can fly except flightless birds (e.g., Ostrich).
They possess beak.
The forelimbs are modified into wings.
The hind limbs generally have scales and are modified for walking, swimming or clasping the tree branches.
Skin is dry without glands except the oil gland at the base of the tail.
Endoskeleton is fully ossified (bony) and the long bones are hollow with air cavities (pneumatic). The digestive tract of birds has additional chambers, the crop and gizzard. Heart is completely four-chambered.
They are warm-blooded (homoiothermous) animals, i.e., they are able to maintain a constant body temperature.
Respiration is by lungs.
Air sacs connected to lungs supplement respiration
Sexes are separate.
Fertilisation is internal.
They are oviparous and development is direct.

Examples : Corvus (Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).