Oh yes, I do understand. 1st - I also went from a D300 to a D800, 2nd - for small birds there is not a lens long enough, and 3rd – I’m defiantly at best an intermediate amateur. I have both the 200-400 f/4 and a 300 f/2.8 so with a TC-2-III I have 600mm @ f/5.6 (IQ very good) or 800mm @ f/8 (IQ not so good). I can hand hold the 300mm with or without a TC for a very few frames. With the D800's resolution high shutter speed is imperative for sharp images – 1.5 to 2 times the inverse of the lens length. Thus 300mm f/2.8 or (if your strong and don't need a new car anytime soon) consider the 400mm f/2.8. It loves TCs and with a 1.4 you're at 560mm @ f/4 with the option of a very fast 400mm. Brad Hill loooves his 400mm. His work justifies his love.
I have used the 600mm f/4 (rented for a trip to Yellowstone) with my D300 and found the lens a bit much for me. I'm not a big man. Remembering my intermediate amateur long lens technique, even on a good tripod using a TC-2 with the 600mm (1200mm) my shots were #### but at 600mm I got some nice shots. My point is, 600mm @ f5.6 using a D800 is a nice solution and not much of a compromise. I do not miss the frame rate, maybe because I've never had it as I shot 14bit RAW with my D300.
Another thought; RRS and the Nikonian store have long lens supports. I have the RRS version and it does make a difference.