The 200-400 can be a great birding lens!
The 200-400 is amazingly sharp up close or as Thom Hogen says "wicked!" I shot this Nene (Hawaiian goose) picture handheld with a 200-400 on a D3. I watched a pair flyby and I observed carefully where they landed. I worked my way over near them, sat down and waited. This one come so close as to be just inside of the close focus limit! For some of the larger avian species, I find this to be a useful birding lens!