You guys seem to be illustrating my point...that with the D800, Nikon certainly has a fine camera, but their lens lineup seems not quite as nice as Canon's. In my opinion, a crop camera will be better for bird photography anyway, making more use of a telephoto len's detail. A 7D or D7000 and a 300 f/2.8, with whatever teleconverters you like, seems like the best choice to me. I think the pixel size is slightly bigger on the D800 than on the D7000, but I could be wrong...they are very close in size...so the main difference is obviously the wider FOV picture, vastly more pixels, and higher cost.
If you are shooting in mid day with very bright light, then something smaller than a 300 f/2.8 would work as well. I haven't tried the Canon 100-400 (it is rumored to be updated soon), but I did try the older 400 f/5.6. It was very sharp with extremely fast and accurate AF even in low light. But it didn't work well at all with my series II 2x teleconverter. Was difficult to get usable shots (at least with reasonable sharpness) even on a tripod...because it was an 800 f/11, or a 1280 f/11 in full frame FOV terms.
Personally I want to try the Sigma 120-300 f/2.8 with OS. The pics I've seen online look plenty sharp in the middle half of the image, on a full frame...so a crop camera might work at least as well, or better. I doubt it will work well with my 2x TC, if it all, but I'll try it.
Last year I also rented a Canon 500 f/4, along with the new Series 3 1.4x TC. I couldn't get it to AF accurately no matter how I set the micro adjustment in camera, with this 1.4x. And manually focusing (via live view) this 1000mm f/8 combo via my series 2 2xTC (on a tripod), I also never could get much sharpness at all...was apparent even via the live view image, not to mention when I snapped a few pictures while in live view (effectively mirror locked). They inspected the lens after I returned it, and said they found nothing wrong...so go figure. It auto-focused almost 100% accurate with no tc's attached (even on a monopod), but it was still never quite perfect. By contrast, the 200 f/2 I rented a few months later, always AF'd perfectly with no tweaking at all, and had extreme sharpness beyond the resolution of my crop camera...and imparted color gradations that made my camera look more like it was full-frame quality. I didn't wind up using it much with the 2x TC, because I liked the wider FOV and how much subject matter it took in...for my Fall landscape shots. Its image stabilization even gave me almost perfect sharpness with a 1 second exposure...on a monopod!
Carl