Wow Caroline - can't tell you how many times this question has gone through my mind 🙂 Didn't realize that the area I moved to was such a birding haven. I'm in Iceland leading some tours with Gary Hart and this weekend back home is a very large birding event with lots of shooters from all over coming in. I am starting to see the joy in it but not sure if I would say I go out specifically to photograph birds. If I did - the 600mm f/4 would be awesome to own. Like you, I own the 400mm f/2.8. Back in my sports photography days I owned the 600mm f/4. It is a razor sharp piece of glass designed to be shot at wide open aperture of f/4, you will not be disappointed. Back in the day, I made a living off shooting 600mm glass wide open. That aperture allows for faster shutter speeds at lower ISO. Let me know what you decide and good luck with your decision.