Hummingbird flight: Two vortex trails with one stroke

As of today, the Wikipedia entry for the hummingbird explains that the bird's flight generates in its wake a single trail of vortices that helps the bird hover. But after conducting experiments with hummingbirds in the lab, researchers propose that the hummingbird produces two trails of vortices -- one under each wing per stroke -- that help generate the aerodynamic forces required for the bird to power and control its flight.
Fuente : http://feeds.sciencedaily.com/~r/sciencedaily/~3/m...