 Feathers on the alula or bastard wing are not generally considered to be flight feathers in the strict sense ; though they are asymmetrical , they lack the length and stiffness of most true flight feathers . However , alula feathers are definitely an aid to slow flight . These feathers — which are attached to the bird 's " thumb " and normally lie flush against the anterior edge of the wing — function in the same way as the slats on an airplane wing , allowing the wing to achieve a higher than normal angle of attack – and thus lift – without resulting in a stall . By manipulating its thumb to create a gap between the alula and the rest of the wing , a bird can avoid stalling when flying at low speeds or landing . 
