Birds are warm-blooded vertebrates.They have hollow bones ,which make their bodies light and help them to fly.They run very fast to get away from predators.Birds lay eggs with hard shells and keep them in the nest.They breathe through their lungs.Other examples are chicken,pigeons,owls etc.
Birds compose a diverse class (Aves) of species, as dissimilar as tiny darting hummingbirds and 8-foot flightless ostriches, with about 9,000 living species known. Generally accepted to have evolved from reptilian dinosaurs, birds share several characteristics with other classes of animals, including a skeletal backbone housing a spinal cord, a four-chambered heart and warmbloodedness. Other of birds' characteristics are unique or essentially unique.
Feathers
Feathers are the defining characteristic of Aves, found on every living species of bird and no other class of animal. Feathers are made of keratin, the same substance that forms hair and nails in other animals and are highly modified scales. Feathers are critical not only for flight but also for warmth and protection against the elements -- and in many species, for males to attract mates. Soft, fluffy downy feathers help keep birds warm, contour feathers streamline birds’ bodies and aid in flying, and flight feathers on wings and tail give the bird loft. Birds shed, or molt, old feathers once or twice each year, depending on the species.
Wings
All birds have wings, although not all birds fly. Nor are wings confined to Aves; bats are flying mammals and most insects have wings. Birds' bodies are beautifully designed for flight, with strong chest muscles and just enough curve to their wings to provide lift. Differences in wing shape provide different advantages to the various bird species. The narrow, sharp-tipped wings of the falcon provide speed; albatrosses soar high on wings much longer than they are wide. Most songbirds have elliptical, evenly shaped wings that facilitate quick, small movements in the tight spaces of their tree homes. Swimming birds, such as penguins and puffins, have flipper-shaped wings that propel them rapidly and gracefully through water.