The alula is a small joint on the bird's wing, similar to the human thumb, with three or four quill-like feathers attached. It is a necessity for low speed flight and maneuverability. The feathers function much like the slats on airplanes by basically increasing the camber of the wing and as such help the bird to land and take-off again.
A small group of feathers attached to the first "finger" (at the bend) of the wing. Their function is to reduce turbulence and drag, and also assist with breaking and steering. Purple Martins have three alula feathers on each wing.