Flight Speeds of Birds in Relation to Energetics and Wind Directions

Abstract

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 t he velocities must be based, relatively elaborate tracking and recording devices are needed. Although many estimates of bird flight speeds have been published (Baker, 1922; Cooke, 1937; Cottam et al., 1942; McCabe, 1942; Broun and Goodwin,1943; Spiers,1945; Meinertzhagen1955; Pearson,1961; Thompson1961; Lanyon, 1962; Schnell,1965; Lokemoen1967; Michener and Walcott,1967), the accuracy of most of them cannot be evaluated because the descriptions of methodology are incomplete. Most estimates are for groundspeed with little or no information on wind velocity. Often the birds were chased by automobiles or aircraft, or were otherwise disturbed, and angles of ascent or descent were not measured. We measured velocities with respect to air and ground of birds in nature by using a double theodolite system. In this technique, the bird is sighted on through telescopes operated by two observers at different locations. The horizontal angles and one vertical angle of the lines of sight of the telescopes are recorded simultaneously and at known times. The position of the bird in space at each time can be reconstructed from these data so that three-dimensionval velocity vectors can be determined.Wind velocity vectors in two dimensions can be measured in a similar manner by tracking helium balloons.