The alula (plural alulae) is clearly visible. The alula is the freely moving first digit on the leading edge of the wing of modern birds. It is the birds “thumb” and is covered by three to five small flight feathers. The alula is commonly referred to as the “bastard wing” – one of several definitions of the word bastard is “of abnormal shape or irregular size“.
Normally the alula is held flush along the surface of the wing and is very difficult to see but when landing or flying at very slow speeds (like hovering) the alula moves forward and upward which produces a small slot on the leading edge of the wing and it becomes more visible.
The function of the alula is similar to that of the slats on the leading edge of the wings of aircraft – to increase the angle of attack of the wing and produce lift, thus helping to prevent a stall during slow flight.