Eric this has been great stuff. I had never looked at the Global site to read what you linked for us before. Since your original post I've thought about my AF results in light of the information, and it would seem to me that it confirms it.
All of my lenses max out as f/5.6 lenses unless I have a teleconverter on the lens. And at this point I only use a teleconverter on my 70-200mm f/2.8 which with a 2X only raises it to f/5.6, and my 500mm f/4 on which I normally don't use more than an 1.4X which raises it to f/5.6, though every once in a while I'll use a 2X which raises it to f/8.
There is no doubt in my mind that my AF results really fall down if I'm using AF-C on the 500mm f/4 with the 2X teleconverter and try to track a flying bird, for example, even a large flying bird such as a great blue heron, or great egret, or bald eagle. The number of frames out of focus is at least 50% higher with the 2X teleconverter on (no degradation with the 1.4X). The lens is on a tripod, and this isn't blur, it's focus.
>For birds in flight, unless you have a medium or fast lens >(f/5.6 or faster) you pretty well need to stick with the >center sensor. Even with a medium or faster lens, there are >relatively few cross sensors. That does not mean the others >won't work with good light - just that your misses are going >to be more frequent.
I quickly found on my own, with the 500mm f/4 and the 2X teleconverter that I had little trouble using single point (center) AF-S if shooting stationary birds, but that when flying, and switching to AF-C, any time the bird was on other sensors than the center sensor, AF was slow and inaccurate.
>I really wonder why Nikon does not allow a menu setting to use >cross sensors only or to display cross sensors in a different >color from regular line sensors. I'd also like to see the >option to only use sensors that work with a given lens. If I >used cross sensors only, AF accuracy should improve.
That would be great. The next time I talk with the Nikon Professional Services people, I'll suggest that. I do get the impression they listen to their NPS members.