“During horizontal powered flight, a bird must flap its wings to generate lift & thrust to overcome gravity & drag. The instantaneous forces on the wings vary during the course of a wingbeat cycle due to time-varying wing planform, degree of flexing the elbow & wrist joints, angle of attack, wing twist, rotational velocity of wings, elastic properties of the primaries, forward velocity, etc.
Birds’ tails also play an important aerodynamic role . . . the tail of many birds could generate as much as a third of the total lift required to support a bird’s weight.”
—the Journal of Experimental Biology (©The Company of Biologists Limited 2001) Flight Kinematics of the Barn Swallow
the barn swallow swiftly dives, swoops, weaves & glides up over the trees then down just above the tips of the wide open fields of grass.
the provided link is from The Cornell Lab of Ornithology’s All About Birds website.