I'll try to mimic the creation of the "Alpha Map" for Piero in GIMP. The description was a little bit confusing for me when I tried to follow it, but I think, I got it now: You copied the mask for fading out the edges of the feather's "Bump Texture" and blended it with that "Bump Texture" in the "Overlay Blend Mode" and repeated this with a second copy of the mask, but this time in "Screen Blend Mode". By this, you got very bright white in the area of the quill and created white feather fibres fading out into black towards the outer tips of the fibres.