Despite all precautions, it is difficult to be sure that no such artefacts biased our results upwards, and the true level of turbulence in the air stream could have been even lower than the recorded values. With the probe in a fixed position, at the centre point of plane 1 we measured the turbulence level at four different wind speeds, from 5 to 20 ms -1 . The lowest turbulence found was at 10 ms -1 . The overhead view shows the location of each survey plane, with the measurements in the centre horizontal row. These are also shown for plane 3, in the middle of the open part of the test section.
Some birds fly spontaneously in the wind tunnel once released by the experimenter.
This does not include all birds and some training is usually necessary. During the migrating season wild caught birds may readily fly in the tunnel after only a few weeks of training. It is of great importance that the birds feel confidant with the observers and the equipment used, otherwise they will not perform well and fly in an unnatural manner. To accomplish good conditions for the birds they are kept in large cages. To keep the birds in good shape and condition they are flown every day in the wind tunnel.