I suppose it depends a lot on what your target is doing. Each method of holding mentioned above has a different centre of rotation, and to an extent, that will determine the ease, or otherwise, of fast, accurate tracking. With Ball-Heads, it's the centre of the ball. Hand-held is somewhere around your neck/head. Gimbal-Heads are usually at the centre of balance in the vertical tilt plane and the tripod's top centre point for horizontal rotation. The downside of anything but hand-held for BIF is that if the bird goes up the back of the camera has to go down, and if using the normal VF so does your head.