http://phys.org/news/2015-06-small-vortex-wing-elegance-birds.html

Then the researchers moved on to the wind tunnel experiments where
they visualized the movement of tiny particles around magpie wings. Here they
observed that a tiny vortex from the alula tip presses the air flow over the
wing and makes the air flow better attached to the wing surface. "For the wing
to fly better, the air has to move closer to the wing. Although the alula is
small, it creates enough vortex to prevent the air flow from being detached from
the wing." Says Dr. Jooha Kim. Measurements of the forces acting on wings in the
wind tunnel proved that indeed the presence of the vortex from the alula helps
the birds in flight.

Thanks to Tom Lanning.