For stationery I always use the 4 spots in the middle that make a cross. That way, with the bird in the center of the viewfinder, it will keep the focus on it. You also have a bit more time to adjust if the bird is still. However, for birds in flight, I always use a wide horizontal bar as the bird is the only object in the sky and as it is moving, you have to move your camera too. This way, the camera will pick it up and hopefully keep it in focus as it moves.