Not having used any of these lenses (except for a few exposures with the 400 /5.6) but having read a lot of reviews my take on the subject is:
I would also say 400 /5.6.
Shutterspeeds for birds should be high anyway, and IS would not make a huge difference. Glass is very sharp, colours are great and contrasty.
Second choise would be 100-400 because of IS, if you would like to use the lens for other things than wildlife. This is also a sharp lens, but lacks contrast compared to the 400 prime.
Third choise, and IMO the best choise if budget allows , is Sigma 120-300 /2.8 OS (there is a new version comming out that should have better AF because of a focus limiter but no reviews yet). That baby takes 2x converters while keeping AF on non pro bodies. You can do the math but I really enjoy writing 600 /5.6 OS so much that I write it anyway. 600 /5.6 OS. Oh and it seems (some of you are going to hate me for stating this) that if you need more focal lenght this lens is a better choise than 70-200 /2.8 + 1.4 or 2.0 extender, unless you compare in the far corners. This is because you are always one stop faster >200 mm and that usually gives better pictures when photographing birds in flight. Also, at approx. 300 mm you have the 1.4x on the 70-200 but no extender on the 120-300, @ 300-400 mm you have the 2x extender on the 70-200 and start to take a hit while you only have the much better 1.4x on the 120-300. Then of course, @> 400 mm the Sigma goes on for another 200 mm up to 600mm, something the 70-200 can not do.
so @ focal lengths of 70-200 the canon wins, on all other they are tied or the sigma is better, then > 400 mm the canon is out. And reach is king for birds.
This argument comes from my studies of sample shots at http://the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=803&Camera=453&Sample=0&FLI=0&API=0&LensComp=687&CameraComp=0&FLIComp=0&APIComp=0
You may not come to the same conclution and I will not defend my statement in endless debate. We all have our favourites and criterias for what will make "the perfect lens". A drawback on the Sigma is its bulk and weight - making it unsuteble for portraits and events where the 70-200 shines.
The Sigma is probably my next lens, btw.
good luck with your choise.