They are warm-blooded or endothermal, bipedal animals.
The epidermal exoskeleton is represented by feathers and a few scales on the legs similar to those of reptiles.
The forelimbs are usually modified as wings and are adapted to flight. The hind limbs are adapted for perching, walking or swimming.
The endoskeleton is mostly spongy, pneumatic, containing air cavities so that body becomes light.
The bones of the skull undergo early ankylosis. Only the lower jaw, quadrate, columella and hyoid are movable. There is only one occipital condyle. The premaxillae are large and form most of the beak. In modern birds there are no teeth.
Most of the vertebrates have heterocercal centra and fusion of vertebrae is apparent. A fused synsacrum if formed by the union of lumbar, sacral and few caudal vertebrae. The terminal usually fuse to form plough shaped pygostyle.
There is well developed sternum with the keel. Ribs are provided with uncinate processes.
The pelvic girdle is fused with sacrum. The ilium is long and extends far in front and behind the acetabulum. The pubis and ischium are directed backwards and there is neither the pubic nor the ischiatic symphyses.
In the hind limb the fibula is incomplete and almost united to the tibia.
In the digestive system, the gullet is dilated into a crop and the stomach is divided into a glandular proventriculus and muscular gizzard. There is pair of rectal caeca between the small intestine and rectum. A cloaca is also present.
The non-expansible lungs are provided with air-sacs which may extent into the bones. The trachea has bony rings. A special vocal organ in the form of syrinx is present at the base of the trachea.
The heart is four chambered with 2 atria and 2 ventricles. There is only right aortic (systemic) arch.
The renal portal system is not well developed.
The RBC are oval and nucleated.
The brain has large cerebellum and cerebral hemispheres and the optic lobes are laterally displaced.
The eyes are usually large and have sclerotic plates and pectin.
The olfactory organs are poorly developed. The auditory organ has large curved cochlea.
The kidneys are metanephric and three lobed. The ureters open into the cloaca. There is no urinary bladder.
The right ovary usually atrophies while the right oviduct is rudimentary.
They are all oviparous and lay large eggs containing large amount of yolk and albumen.
Fertilization is internal. The egg posses double shell membrane and calcareous shell. During development the embryo develops extra embryonic membranes like amnion, allantois and yolk sac.