i am currently using 1D4 with a 800 5.6 and my wife is using 7D with 400 5.6.
weight is certainly an issue and as with budget, however, trying to remain on the topic of focal length, you should also take into consideration what body you will be using.
1DX = full frame
1D4 = x1.3
7D etc = x1.6
As for Nikon, it's either full frame or x1.5 (i think) ..
i have been tempted to get the new 1DX however, until today, it was NOT compatible with x1.4 teleconverters, which means that for real focal length, i lose x1.3 coz 1DX is full frame, whereas 1D4 is x1.3 AND ... i am unable to attach a x1.4 on my 800 5.6 ..
if you want light weight, handheld, consider 300mm 4.0 with x1.4 converter on a 7D or 60D (giving an additional x1.6) .. the package will give you 672mm, if you are using a 1D series body (except 1DX), you can even use x2.0 converter, giving you, 780mm with Image Stabilizer.
alternative to this package, would be a simple 400mm 5.6 on a 7D or 400mm with x1.4 converter on a 1D body .. 640mm and 728mm respectively without IS.
in my opinion, the 2nd package will give better image quality over the 1st, however the 1st option does give you IS which is handy but not essential (as you are likely to shoot with high shutter speeds and IS isn't as useful as you originally thought....
birders will never get enough Focal Length! even while i have a 800, i am still looking for more when I get to MP etc .. having said that, if forest birds, mobility, price is the priorities, then i think the above two options are more than adequate!
the benefit of the above two lightweight options are that you can probably get away with handheld or with just a monopod, 500 or 600 will certainly mean a gitzo series 3-5 tripod plus a big ballhead which can weight as much as your lens!!
