I'm late to the party but for what it's worth......

I would agree with Peety .....ultimate would be the 600 f/4L and the 400 DO II .... fabulous IQ and both will take the 1.4 or 2.0 tele-extenders well but you might want to do an AF micro adjustment to really maximize performance. You probably won't be disappointed with the 500 though, it's been my "weapon of choice" for years. I would strongly recommend you have the tele-extenders available when birding.