Originally Posted by
rpcrowe
I am familiar with both the 400mm f/5.6L lens and the 100-400mm f/4.5-5.6L IS II lens.
The 400L is sharp and quick focusing and the lightest weight of the batch but, this lens doesn't have Image Stabilization. Sure, most of the rime for BIF, you will be shooting at a high enough shutter speed so that he lack of IS won't be that important. But, if you want to shoot in many other venues, IS would be a great addition.
The 100-400L II is no lightweight. It weighs right about what a 70-200mm f/2.8L IS weighs. The focusing and the image stabilization are superb as is the IQ! All in all, this is a truly excellent lens. I can easily hand hold it due to the great IS capability.
Another thought might be a 300mm f/4L IS lens with a tele converter added. That would give you focal lengths of 300 and 420mm with a 1.4 extender. I believe that there are third party 1.5x power teleconverters available. This would give you a 300mm and a 450mm combination. The 1.4x TC worked quite well with my 300mm f/4L IS lens and I really didn't notice any appreciable degredation of AF or IQ.