James,
Roger has the fast glass with enough reach so that he doesn't have to use telextenders. My longest lens is the 70-200 so I often use it with a 1.4X converter and I've had no problems with that combination catching BIF with my 7D. I wouldn't use a 2X converter because with my setup, I'd lose autofocus ability.
My technique is the same as getting ring candids of dogs that are showing or shooting dog performance events: I use the center focus point only, AI Servo and fast frame rate (8 fps with the 7D). I don't always get all the frames in focus but I'd estimate between 90 and 95% of them are. The key is to lock onto your subject with that first frame and track them so that the center point stays on them. I do not use a tripod. Everything is handheld.
Two samples:
For me, the hardest tracking is when the subjects are coming directly at you. I have far less difficulty (as does my camera) tracking action that is moving laterally across the frame. This is a shot of our new kid, Jimmy, on a retrieve with Moxie in hot pursuit
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And here's a shot that is more germane to the conversation, a White Ibis in flight.
Hope that helps.
Jim
