2 vortex trails with 1 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 at the University of California, Riverside 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://www.eurekalert.org/pub_releases/2013-02/uoc...