Alula

The alula, or bastard wing, is a small, feathered projection on the anterior edge of the wing of modern birds. It is actually the thumb of the bird, and has been modified into an aerodynamical control device not too dissimilar from the forward slat on an airplane wing. It is held open in slow flight to prevent the bird from stalling. Usually flush against the wing, the alula may be moved to modify the shape of the wing or provide a slat for direction of airflow, often to avoid stalling while landing. It has been found in all modern birds since Eoalulavis hoyasi, a primitive bird from the mid-Cretaceous, 115 million years ago.