Oh I've eyed that lens for some time! I'm addicted to zoom though I get nervous that having a fixed lens makes it harder or something. I was thinking the 70-200mm for my first pro lens but I'm weighing whether or not to get the IS because it's only $700 without it!
The 400mm prime is good for me because I need the reach. I am rarely too close to the birds (although I do also carry a second camera with a 55-250mm lens just in case). Since you don't photograph small birds in the wild very often (judging from your gallery) then a 70-200 might be a better choice. As far as whether or not to get IS, that depends on your usage. For me, because I use fast shutter speeds anyway, the lack of IS is not a problem. Also, if you do a lot of shooting from a tripod, then you wouldn't need/want IS. The other big factor, I think, is image quality. Get the best quality lens that you can afford.
Thank you for all those tips. I don't do a lot of wildlife shooting of birds simply because I don't have a long enough lens! Although I have photographed little wild birds like the kinglet, it was a rare shot where the bird was really close and I didn't need a lot of reach. This was taken with the 70-300mm tamron, and only at 96mm! Little guy just wasn't afraid of me. (Although I am very quiet and stand as still as possible when watching birds.)
You definitely need to dress warm. I have a pair of gloves (fleece ones that I bought at the dollar store) which I cut a couple of slits in, so that I could expose the end of my index finger and thumb. That way I can keep the gloves on and still operate the shutter and do back-button focusing.