I find it helpful to create another layer for shadows etc, if I decide that there needs to be more values in an image. Here I have separated two sections of the wings, with a "darken" layer for the bottom part. I find it doesn't really matter what colour you use to darken an image. I prefer to use black, because I associate it with shadow. I guess it's a childhood thing :)

I stencil on the outlines of the feathers in the mid section with a fine white brush, and dodge a few slanting strokes to show the direction of the feathers. On a separate layer, I stiple a few darker patches - along the bone of the wing, and just for variation over the rest of the wing.

Retouching Drawings Part 7