Kinematic and aerodynamic parameters in tethered flying siskins (Carduelis spinus, Passeres, Aves)

Summary

Stationary flight of siskins (Carduelis spinus) was achieved by tethering them in a harness of yarn strands fastened to a metal rod (Fig. 1). The wing-beat frequency and the wing-stroke angles were recorded by miniature movement detectors, i.e. small sensing coils glued on the armwings. Kinematic parameters were also recorded cinematographically. The aerodynamic parameters, lift and thrust, of the tethered flying birds could be measured by mechanoelectric transducers in a flight balance (Fig. 2). The frontal air current of a wind tunnel (Fig. 3) stimulated the flight movements, and generated a drag which opposed the thrust. When these two forces were brought into equilibrium, the wind speed equaled the flight speed of the bird in relation to the air.

Tethered flight begins with a continuous flight which changes to the leaping flight in which flapping intervals alternate with pauses (Fig. 4). The leaping flight lasts up to two hours. The wing-beat frequency is about 19 Hz both, during the continuous flight and the flapping intervals of the leaping flight. However, it is only about 9 Hz during the leaping flight if the pauses are averaged in, too. The flight pattern as well as the kinematic of the wing movements (Fig. 5) of tethered flying siskins resembles those of free flying small birds. Whereas just after the start the generated lift is greater than the body weight of the siskin, these two forces are approximately in equilibrium during the following flight periods (Fig. 6). If, in addition, the thrust of the flying siskin compensates its drag, the aerodynamic situation of this tethered flight equals that of horizontal, nonaccelerated free flapping flight.