Rare White Morph Gyrfalcon - D800e 70-200 VR II and TC 2.0 III

Lower ISO is better, but as pointed out, the D800 is awfully good.  The big issue with higher ISO is that you lose some dynamic range.  But your picture doesn't have a whole lot of DR - whitish bird, grey background.  So you can push ISO, deal with noise, keep up your shutter speed and not lose much in post.

Next time convince the bird to hang out against a much darker backdrop with the light coming over it's right shoulder.....

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RG www.lostrange.com