A streamlined body is a special kind of body shape that is tapering towards both the ends and broad in the middle. This unique shape helps birds to fly in air with less resistance.
Forelimbs are modified into wings for flight.
Skin is dry without glands except the oil gland at the base of the tail.
Endoskeleton is fully ossified, and the long bones are hollow with air cavities (Pneumatic).
Bipedal locomotion, forelimbs are used for flying and thus hindlimbs left for locomotion. Hindlimbs balances and supports the entire weight of the body.