If your sole purpose is birds I'd get the EF400/5.6L for it superb glass, fast AF and portability. It's a superb tool for flight shots hand holding! IS is a non issue in this case.
For general nature shots, EF300/4L IS is the choice. Adding an EF1.4x converter you then get the flexibility to shoot closeup too as the MFD is only 1.5m (4.9 ft).
EF100-400/4.5-5.6L IS is flexible of all. But it's push-pull construction and sample variation is too much due to complex lens design. If you're lucky you get a sharp copy.
Hope the above info is useful. Good luck!
