EF 600mm f/4 IS II
I also had several opportunities where I was able to approach perched kites for close-up shots. Needless to say, the AF is accurate and stable for static subjects (note that because action can happen at any time I never switch AF to single shot). The perched frames came out very sharp and I am impressed with the level of field sharpness I can obtain with the EF 1.4X extender III at wide open aperture (f/5.6). In the past, I have had great results with the original EF 500mm f/4L IS and 1.4X extender, but at times I would stop down to f/8 just to squeeze out a tad more sharpness and contrast. With the new lens I would only stop down for depth of field. Click here for a 100% crop from RAW of the image below taken at f/5.6 for a close inspection of details. Given the very good performance of the new lens with the EF 2X TC III at f/8 I can see this combo being useful for static frames. Unfortunately recent Canon pro cameras have dropped support for AF at f/8, but on the 1D MKIV, 1200mm f/8 is now a valid proposition.
It is worth noting that the new AF module used in the EOS 1DX and the EOS 5D Mark III cameras, all AF points remain active with the EF 600mm II plus EF 1.4X extender combo. This is in contrast with the EF 800mm f/5.6L IS where the outer peripheral AF sensors are disabled. The peripheral sensors are useful when shooting off-centered perched birds. Note that given the narrow DOF, focus-and-recompose technique will result in out-of-focus images at such focal lengths.