The previous post about setting one-shot vs. servo vs. AI is, I believe, not the best way to setup these cameras. Using custom controls I have reassigned the AF-ON button for use as the "start focus" command, and I have disabled focusing from the shutter button.
So now here is how it works: Put your cross hairs on where you want to focus, and press AF-ON and release it. Then move the camera for framing. Now press the shutter button any time you wish to trip the shutter. The camera will never try to re-focus, and so you will not waste time and possibly mis-focus by using the standard setup.
Now if you select A1 or Servo there is another advantage. If you really want the camera to continuously focus, just press and hold the AF-ON button. So I think this setup gives you added capability.
Many of my serious birdie photographers friends use this technique and love it.