I'm having trouble figuring out how AI Servo mode works on my Canon 70D from the instruction manual. If I use center point focusing and press the shutter button half way to focus on the subject and the subject moves, does maintaining focus on the subject require that I move the camera so that the subject stays on the center focusing point? Would this work differently and/or better if I let the camera choose the focus point (I've been trying to use AI Servo to photograph birds, so this is probably not an option since the camera could decide to focus on a branch, but could this work on other types of subjects)?