If you have used the D700 a lot, then the D600 will feel kind of like a downgrade in the way it handles. I have tried it out in the store, as I was eying the D600 for travel use, but it just felt too different from the D700 and D800. You are certainly right, in image quality and ability to crop, the D600 has the advantage. I agree with Stan, the D800 combines the handling of the D700, with even more MP for your birding use and reach, at a price of course. But if you will be using the camera for many years, better get the right one.