The pen of the painter comes from the wing of the woodcock. It's a small sharp pen shaped spearhead, flexible and rigid at a time. It is also called brush or surrémige. In the Middle Ages it was used to achieve the illuminations.
Each feather is unique in the way we behave. You have to adapt to each one. We must "feel" its orientations, its inclinations because it leaves no room for error. Because of its fragility, each feather is no reason to make a restrainte surface of a drawing. Under each work done, or you will find the feathers used in the making of the work.