http://www.nytimes.com/2009/06/16/science/16obseed.html?hpw

Dr. Lentink, with Michael H. Dickinson of the California Institute of Technology and colleagues, report in Science that the wings generate a leading edge vortex — a spinning horizontal tunnel of air along the wing — as they descend. This vortex is stable, Dr. Lentink said, because it has a low-pressure core, reduces the air pressure over the wing, causing the wing to be sucked up. “It really increases the lift,” he said.