Best use of AFC mode?

I'm trying to learn the D300 AFC mode, and I understand that if I press the AF on button and hold it, focus will be continuous, and if my subject moves, will follow the subject. Mu question is: how much do I need to pan to keep the moving subject covered by the chosen focus point? If the subject moves but remains in the area of the focus point array, is the camera following it, even if I don;t pan to keep the particular focus point over the subject?

If this is the case, it would seem that the 51 point choice would almost always be the best for birds.
Sarah