Really nice images of birds in flight, Richard. Great colors compared to what I've been able to capture, as well as great focus on birds in motion. I especially like the sense of change in flight direction you were able to capture in the hawk photo.
Your photos were captured with very different techniques, as you say. They illustrate a point I wanted to bring up about dynamic-area AF with the D800. For your Shellduck, slow panning example, you may be able to focus on a particular area of the body, even the head. For this situation, I prefer a smaller, e.g., 9-pt area AF. For the hawk example, flying faster with more direction changes, I've found that 51-pt area AF does surprisingly well, just in acquiring the target. In coming from the D7000, I was used to using small areas for AF to have tighter control of the focal points used. With the D800, I've tried 51-pts when simply acquiring the target is the primary challenge with better than expected results. This works better of course when the subject stands out against the background.