You need to be shooting in continuous AF mode so that once focus lock is obtained, the camera maintains focus on the moving bird.
How many AF points is the big question. If it's a falrly large bird and not too far away then you may be able to use just a single AF point as long as you can keep it over that mving bird.
For smaller birds or where there's more range involved you 'd do better to enable all AF points. When enabling all AF points then you need to first get focus lock with the central AF point, after which theoretically focus is passed from one AF point to another as the bird moves in the frame in relation to that pattern of AF points. In practise the active AF point only has to fall briefly on a part of the bird with insufficent detail or contrast for the AF system to function, for the system to lose focus momentarily and result in a blurred image. Even with the very best cameras you're going to get the odd duff frame or few - BIF shots aren't easy!
Generally speaking birds coming towards you or going away will be more difficult than those crossing at an angle, and birds against the sky will be easier for the AF system than those against a background of trees etc.
