I shoot with a D800 and the Sigma 150-500. In terms of image quality & high ISO performance from what I read there is not much difference from the D700 to the D800 in this regard. I can't confirm this I have never shot with a D700. I am very pleased with the combo I am now using, I came from a D300 & 200-400 combo. Here is a thread I posted a few weeks ago with some images from the new combo. IMO the most important thing in capturing excellent images of birds in flight is enough light & being close enough to fill at least 1/3 of the frame.