Birds and bats approach flying differently. When flying at slow speeds, birds easily can separate the primary feathers on their wings to allow air through, which makes their upstroke aerodynamically inactive. The membranes on a bat wing, however, mean that air is pushed on both the upstroke and the downstroke.To learn more about the role of bat wing movement and how the physical differences between bats and birds affect flight, Anders Hedenström of Lund University in Sweden and colleagues from the University of Munich and from Max Planck Institute for Ornithology in Seewiesen, both in Germany, as well as from the University of Southern California in Los Angeles used imaging to study a small nectar-feeding phyllostomid bat species, Glossophaga soricina.To study bat flight, the scientists filled a wind tunnel with fog and captured images of the movement of fog particles as the bats flew in the tunnel’s test section. To calculate how the fog particles moved required a digital particle image velocimetry setup, which, for this work, consisted of a dual-head double-pulsed Spectra-Physics Nd:YAG laser operated at a repetition rate of 10 Hz and a Redlake CCD array camera that captured image pairs separated by 200 to 300 μs.As detailed in the May 11 issue of Science, the study revealed that, at slow speeds, bats gain lift by flicking their wings backward during the upstroke. It also showed that each bat wing generates its own vortex (see figure). The arrows show the air movement created from the wings. Another finding was that, during the turn between upstroke and downstroke, the outer part of the wing generates negative lift and the inner part of the wing positive lift, so that different parts of the wing produce extra vortices in the wake. The researchers believe that the information revealed by this study — besides providing insights into two evolutionary pathways taken to solve the problem of flight — also might aid in designing flying devices.