The nankeen kestrel is one of Australia’s smallest raptors, but one of its fastest, too. It is not particularly easier to photograph in flight because it is so fast. The answer is to employ a lens with rapid autofocus, a camera with advanced autofocus and tracking, and to accept that one must crop the image rather heavily.
This image shot with the Sony 135mm GM lens on a Sony A1, at ISO 100 f/1.8 1/5000 using bird eye autofocus (check the focus on the eye below). The image was cropped down to 5700 x 3800 from the 8640 x 5760 of the full frame.
Here is the full frame (and the crop) so you can look at the details (click to enlarge):