Presence of feathers the characteristic feature of birds
Most of them can fly except flightless birds (Eg:Ostrich)
Forelimbs are modified as wings
Hindlimbs 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)
Digestive tract of birds has additional chambers, the crop and gizzard
Heart is completely four chambered
Homeothermy (maintains constant body temperature)
Respiration by lungs. Air sacs connected to lungs to supplement respiration
Sexes are separate
Fertilization is internal
Oviparous
Development is direct
