In progress - Owl drawing

[amberdreams]

I'm trying to draw this Barred Owl for my old friend Mal, who is a big fan of owls in general. I know I need to sort out the right eye, but any tips on enhancing the feathers and other details would be gratefully recieved. Also how can I get the darks nice and dark? So far I've used 2B and 6B pencils - I do have up to 9B.
This is the reference pic (this chap takes fantastic bird photos)