Once you lock on to a target the camera focus will follow that subject as long as you have the AF button pushed down or you are holding the shutter release partially depressed. But this is true for all the various focus point modes provided you have AF tracking enabled.
The 51 focus points are not meant to say you can push your little square point around in 51 different places, rather it refers to your camera using 51 different points to acquire a solid focus and will use all the input from these points to insure you obtain a perfectly focused image. The various focus points talk to one another when determining the best focus and take into account curves and changes in structure of your subject.
But with birds I would suggest you leave the focus point in the center and latch onto the bird and follow it with your shutter release partially depressed and then take your photo when you're happy with the image.