Automatically tracking a bird in flight isn't a super important thing. Realistically, what happens is that if the bird is small enough in the field of view where tracking is useful, the photograph won't show enough pixels of the bird to matter. The shot you really want has the bird taking up as much of the frame as is possible, and in those cases, you have to manually track (point lens at) the bird, or it will be out of the frame in a fraction of a second.lens look very interesting but I wonder when/if Canon will be R series body that be able to track a subject (bird in flight, football player and shot at 12 FPS. I understand that this is not easy do on a mirrorless camera.
In my opinion, the more important thing for really serious BIF folks will be, if you take a RF super telephoto and stick a 2x teleconverter on it, will the future, super-pro RF body focus on a single AF point as quickly as a DSLR, and when you manually track the bird, will the AF system keep the subject in focus, especially if it is flying towards you -- for example, launching off a perch.