Origin of Avian Flight - Flight Characteristics

Flight Characteristics

For flight to occur in Aves, four physical forces (thrust and drag, lift and weight) must work together. In order for birds to balance these forces, certain physical characteristics are required. Asymmetrical wings, found on all flying birds with the exception of hummingbirds, help in the production of thrust and lift. Anything that moves through the air produces drag due to friction forces. The aerodynamic body of a bird can reduce drag, but when stopping or slowing down a bird will use its tail and feet to increase drag. Weight is the largest obstacle birds must overcome in order to fly. Flying birds have evolved reduced weight through several characteristics. Pneumatic bone is hollow or filled with air sacs, reducing weight. The loss of teeth, gonadal hypertrophy, and fusion of bones also reduce weight. Teeth have been replaced by a light weight bill made of keratin, and chewing occurs in the bird's gizzard. Other physical characteristics required for flight are a keel for the attachment of flight muscles, an enlarged cerebellum for fine motor coordination, and a furcula, which enhances skeletal bracing for the stresses of flight.