I'd probably just-- if you told me where the light source was coming from and sent me the sketch, I could lightly outline where the lit parts would be (without wing shadows; I'd leave that part to you.) I wouldn't go overboard, and I definitely wouldn't mess with any part of your sketch, and I wouldn't be hurt if you decided to change or correct anything, or approach it a different way entirely :)

If you don't like the idea of me drawing over your sketch, I could try describing it in words, but I think the first idea is more efficient.