The Nikon 500mm vr is an amazingly precise device if used judiciously with a tripod with "tripod on vr" activated. You can get 50% keepers against about 10 to 20% on small birds with the Sigma 150-500. Of course you pay the price... in expense and weight. The F4 opening throughout and speed of the focus is impressive. Kudos for nikon...Of course, the D700's FX reduced reach does not favor in flight small bird shots.