• Widely distributed. Warm-blooded (endothermous) or homeothermous vertebrates.
• The body is fusiform covered with feathers. Scales cover the hind limbs.
• Forelimbs are modified into wings. Whereas hind limbs consist of usually 4 toes and adapted for perching, walking or swimming.
• Adapted for aerial life. Lungs are the respiratory organs, they are connected to thin walled air sacs.
• Skeleton is light and bones contain air cavities (pneumatized) skull is monocondylar as in reptiles.
• Voicebox is called syrinx which is situated at the base of the trachea.
• Heart is perfectly 4 chambered. There is a single aortic arch on the right side only. RBC are nucleated and biconvex. 
• Kidneys are advanced type that is metanephric. Uric acid is the nitrogenous waste. Sexes are separate. Ovary is single in female. Fertilization is internal. Oviparous laying eggs. Eggs contain much yolk. Eggs are covered by shell. Eggs hatch after a period of incubation by parents. Birds show high degree of parental care.
• Jaws are modified into beak. No living bird possesses teeth. Class Aves divided into sub class Archaeornithes and Neornithes. Living Birds are included under Neornithes which are classified into Ratitae and carinatae. Ratitae are flight less birds whereas carinatae are flying birds.

Examples – snakes, lizards