That's one amazing capture. Such a skill required to nail a shot like this, but as you said the 2N (and the bare 500) helps. I like the matched colors between the bird and the background too. Possible I would move the bird slighly down and to the right when cropping. Thanks for sharing.
I doubt if there are many flight shots of this bird floating around. The near wide open 500 f4 blurred the background to a creamy bokeh, while the fast AF of the 1DM2 is superior to the speed of the bird.