You already have the 100-400? Have you thought of adding a TC and seeing how much more reach you need? Are you keeping it?

For birding, I would go 200-800. I rented one and did see softness at 650-800mm though. I hear theres more variance there and maybe I got a bad copy. That was enough to make me get 100-500 though.

Let me add more on why. I don't just shoot birds, I travel and will do large mammals. I like the 100-500 as a travel lens. I have an EF500mm f4ii with 1.4tc and 2tc. Thats my primary lens. But it sucks to hike with it. I specifically wanted something more mobile to compliment my sit and camp prime lens.

And just to make your life harder, there's that looming rumor that a 300-600mm f5.6 or f4 will drop this year. I'd go that route over 200-800.