Despite the oft-quoted assertion that engineers have shown that bumblebees can't fly, the basics of why flying animals don't fall out of the sky is now mostly understood. But the details of flight – the effects of changes in wing shape, motion and structure, in fact almost everything that interests comparative biologists – remain largely a matter of conjecture, mostly because it has proven enormously difficult to do quantitative aerodynamic measurements on living, flying animals. The problem is that everything moves too fast in air: insects and birds fly quickly and beat their wings even faster. In the November online edition of Experiments in Fluids,Geoff Spedding, Anders Hedenström and Mikael Rosén offer a solution to the challenge. They describe a modification of a standard technique, called particle image velocimetry (PIV), to quantitatively measure the air flow behind freely flying birds in a wind tunnel.

PIV works by tracking small reflective particles floating in air or water. But instead of tracking many individual particles, which leads to many errors,it tracks the average movement of groups of particles, which is much less error-prone. In a wind tunnel or flow tank, these particles constantly move past the camera; it's the changes the animal makes in this background motion that are interesting. At low velocities, separating these disturbances from the background flow isn't hard, but high velocities produce two problems. First, you often need larger groups of particles to measure high velocities,limiting the number of measurements; and second, the faster a bird flies in a wind tunnel, the smaller the disturbance becomes relative to the wind speed,limiting measurement accuracy.

Spedding, who started out attempting to perform similar measurements in bird wakes 20 years ago, has adapted the PIV technique for the high velocities common in flight. He uses two matched cameras, one upstream of the other but both behind the bird. This configuration lets him measure only the animal's wake, without the background flow. It's a simple idea: the distance between the cameras is matched to the wind speed, so that it's equal to the distance particles travel, on average, in the time between two video frames. Particles in the upstream view will then appear in the same position in the downstream view one frame later – any net motion, therefore, must be from the bird. This simple idea increases the resolution and accuracy of PIV measurements at any background wind speed.

Their preliminary results for a slow-flying thrush aren't surprising: the bird only produces force during downstroke and has an asymmetrical wake, as previously observed, with higher speed flow produced at the beginning of downstroke than at the end. Despite the lack of surprises for the thrush,further high-resolution studies of other birds may begin to solve the problem of why different birds fly differently.

Spedding, G. R., Hedenström, A. and Rosén, M.(
). Quantitative studies of the wakes of freely flying birds in a low-turbulence wind tunnel.
Exp. Fluids.
DOI 10.1007/s00348-002-0559-8.