How do birds fly?

A bird’s wing is curved on top and angled into the airflow, so air moving over it creates lift, just as with an aeroplane wing. On the downstroke the wing pushes air down and back, producing both lift and forward thrust; on the upstroke many birds partly fold their wings to reduce drag. The flight muscles, especially the large pectoral muscles anchored to the keel of the breastbone, can make up around 15 to 25 percent of a flying bird’s body weight. Many bones are hollow and reinforced with internal struts, keeping the skeleton light but strong. Birds steer by twisting their wings and tail, and large birds such as albatrosses and vultures save energy by gliding and soaring on air currents.