Your original comment to which I replied said:
That implies that you don't think the camera needs to be able to track a rapidly moving subject. Then you responded to my comment with "... why would AF not be tracking it?"
Which is it?
How does a 15 frame rolling buffer reduce the need for rapid tracking AF with a subject that is moving rapidly? Unless you're happy with a random lucky one frame out of many in which the bird is at the correct distance, but probably not in the best 'pose', for the constant focus distance of the entire series? What would be the point of that?
Shooters who want fast frame rates and Servo AF expect many/most frames to be in focus because the camera is actively tracking the subject. The hit rate for cameras such as the Canon EOS 1D X Mark II and Nikon D5 at 12-14 fps are remarkable. No mirrorless camera, not even the Sony α9, can come close. Cameras such as the Nikon D500 or Canon 7D Mark II aren't far behind them at considerably lower prices. When the α9 is shooting at 20 fps it can't do AF-C (or save 14-bit raw files). To get AF-C and 14-bit raw, the camera slows down considerably and is effectively no faster than a 1D X Mark II or a D5 when tracking the subject every frame, and the Sony α9 does not get near the 'hit rate' that the two mirror slappers do. The only way to get 20 fps with the α9 is to accept no AF between frames and 12-bit raw (or 8-bit JPEG).