Well you would lose reach for sure. Its about pixel density. The D7000 is about the same pixel density as the D800. So shooting with D600 you would put less pixels on your bird. So if you cant fill the frame you would get less detail with the D600.Now other factors also come into play such as focus. The D600 is supposed to be an improved D7000 focus module. Plus iso and noise could also make the D600 better than D7000. But if your strictly a bird shooter I would keep your D7000 and wait a while longer to see if a D400 appears. No idea if one will happen at all or when BUT if one did come out I would bet it would be at least 20 meg with the 1.5 crop so its pixel density would be the highest from nikon plus most likley have the same focus as the D800 does and shoot at maybe 8 fps. Now that would be the best bird camera you could get.