Hi Jason, if you want a birding only lens the 400mm f/5.6 is far superior to the 300mm f/4. The AF is quicker and you don't need as many extenders, extenders GREATLY affect AF also. Bare it's sharper and handles extenders better. The 300mm is more versitile with it's IS and wider aperture but since you are stacking extenders you would be better off with the 400mm.


John.