That's a beauty of a flight shot, and the lack of sunshine may have done you a favor. Good detail, except for a bit of motion blur on the wings that doesn't bother me. You might tease just a bit more color out of the head. Just a little large in the frame for my personal taste; were it mine I'd add a bit of canvas all around, and give it a 1 px border.
Considering the lack of sun I am impressed that you got this one so nice and sharp. Great HA and a perfect look at the golden eye. Well done William
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Will this is super, boy your images are really good - looks like you have got to grips with the 1DX well and truly!!
(You will be looking at the 500mm MKII soon)