What are the characteristic features of birds ?

The three classes of living animals which have made a success of flying are birds, bats and insects. Of these the birds are the most adapted for life in the air. Research has revealed that birds evolved from reptiles.
Birds Most flying birds have lightly built, stream -lined bodies which enables them to move effortlessly in the air. Most flightless birds are however heavy and large.
One of the characteristic features which distinguishes birds from other invertebrates is that they possess feathers. These form a covering for keeping the bird warm and dry. The feathers overlap each other and point backwards for smooth flight.
The jaws of the bird forms a beak. The shape of the beak depends upon their diet. In the same way, the claws of the birds are also adapted to their diet and lifestyle. They are sharp, hooked, strong, webbed or blunt.
The neck of the bird is very flexible thereby helping it to preen feathers on all parts of the body. The nostrils are two small slits on the upper part of the beak, but their power of smell is poorly developed. They however have keen sense of sight and hearing.
Compared to other animals, birds have the wonderful capacity of adjusting their eyes very quickly from a distant object to a nearby object.
Their ears are two small little holes on either side of the head. As they are covered by feathers, they are not easily visible. However their sense of hearing is very sharp and they use it for hunting, self-protection and mating purposes.

Birds are warm-blooded creatures. They do not hibernate. However several species migrate to warmer climates during the cold months. They cover long distances across continents, some of the migrating birds are the swallow, goose, duck, swan and tern.
The smallest bird is the bee-humming bird, while the largest is the ostrich which weighs 180 kg.