Keep your 80-400, it will work just as well on a DX or FX DSLR Body as the Nikon bodies all have the f bayonet mount. Get the new one if you want, but I think it would be a waste as nikon lenses typically keep their quality and are considered an investment. For birding in good light either the 7100 or 7200 would do, but the expeed 4 sensor on the 7200 is the latest generation and is supposed to function a bit better in lower light, higher iso will have less noise, and the focus should be a bit faster. I am getting a d810 in a week or so, next time Nikon has a sale, and am planning on getting some sturdy older nikon primes to pair it with.