Birds can be tracked in the field using the different methods described above. However, for experiments under controlled conditions, a wind tunnel is required. A wind tunnel creates a smooth (laminar) airflow in a test section where birds are trained to fly (Figure 7.5). There are a few wind tunnels dedicated for bird flight research in Europe and North America, but especially two new wind tunnels of recirculating design at Lund University, Sweden and the Max Planck Institute, Seewiesen, Germany, give very low turbulence in air flow (Figure 7.5; Pennycuick et al. 1997). This is important in order to generate a natural situation reflecting flight through nonturbulent air. If one is interested of studying the
effect of turbulence on flight, turbulence can easily be created by inserting nets or objects upstream from the test section.
One important role for wind tunnels is the validation or testing of flight mechanical theory, which among other things, predicts a U-shaped relationship between power output and flight speed. Measurements of metabolic rate, using a respirometry mask or doubly labeled water, have often been used for evaluating flight mechanical theory, but this measures power input. These measurements are only valid if the conversion efficiency of fuel energy to mechanical work is constant at all speeds, which might not be true. Direct measurement of mechanical power output is difficult, but can be made by inserting a strain gauge and measuring the force applied to the humerus by the flight muscle (Dial et al. 1997; Tobalske et al. 2003), variations in the vertical acceleration of the body combined with wing-beat kinematics (Pennycuick et al. 2000), or from estimating the impulse associated with vortex wake structures.
The wind tunnel can also be used for studying wing-beat kinematics and flight style by using high-speed video cameras.
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