RECENTLY it has become possible to measure accurately the power expenditures of birds flying freely in a wind tunnel (Tucker, 1968, 1969). The measurements show that power expenditure is influenced by the air speed and the angle of flight, which may be level, ascending, or descending. As the air speed and angle of flight in a wind tunnel are chosen by the investigator, we were interested in determining if birds flying in natural conditions choose air speeds that minimize their power expenditures. Accurate measurements of air speed and angle of flight in nature are difficult to. make. The velocity vectors of both the bird and the wind must be measured, and then the motion of the bird relative to the air must be determined by vector addition. Both vectors may change in time and space. As the unaided human eye usually is incapable of measuring the distances and angles on which accurate measurements of the velocities must be based, relatively elaborate tracking and recording devices are needed. Although many estimates of bird flight speeds have been published
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