Follow action requires a gimbal and big glass requires a BIG gimbal. They do this all the time in motion picture industry but that equipment costs as much as a house.
I think this is not true. Following action does NOT require a gimbal and it is even hampered by a gimbal or any other kind of static support.
So why anybody uses it? To hold the weight and to reduce the vibrations. I'm not an expert but I bet that any BIF photographer would happily drop any gimbal/tripod/monopod any day if just they could. But up to the very recent years this was simply not possible. Bean bags and tripods are great but for different kind of animals.
In motion picture, for fast action shots, they use complex body mounted rigs to offload the weight on the shoulders or hips allowing maximum movement freedom (or some kind of dolly if the shot allows). But cinema is all scripted so you know exactly where the subjects are going to be.
Unless you plant to shoot near a mountain cliff where birds glide in a very predictable near static pattern any kind of static support, even a monopod, will constraint the movements you can make. And a fraction of degree of error or delay is enough to completely loose the target.
I usually loose the bird after I rotated my hips 90 degrees to one side and my rotation stiffens or get too slow. Or I move a foot and that is enough to shake the camera and loose the target, and the sky is all the same so you do not have any reference to reacquire it (shooting with both eyes open helps a little, or the Olympus ee-1 I suppose).
Consider that you want to shoot the birds when they are close, not just a small black dot in the frame. It is very easy to follow a distant bird, and very hard to follow one close enough to "fill" even 1/9 of the frame. And (usually) you want them facing you, front lit, not just the tail silhouette.
My advice: take your current lens, a 100-300 I think, go out and try to use it in manual focus. Do it in good day light. The "small" aperture won't make any difference and will even help. Try to take a few shots. A different lens won't make it any easier. A 1200mm equiv. lens would make it impossible. A gimbal will make it worse. Don't trust me, just try.
