Thanks Bent, I am just a learner, and found some useful tips in the following thread:
https://www.dpreview.com/forums/post/67691713. There is mention of a video by Steve Perry. I limited focus modes to Back button focus with AFC mode of course and under controls (f2) I currently have video record button programmed to cycle though AF area modes limited to Dynamic area (S), Wide-area AF (S), Wide-area AF (L), 3D and Auto Area. (I left all of them available in the shooting menu, as the selected ones need to be selected both places for the switching to work.) This allowed changing AF area mode on the fly so to speak. Subject detection of course to Birds, and with a note that it is not active for Dynamic area, so that would if anything be used for stationary birds.
I also have fn2 set to switch between FX and DX mode. If frame coverage of the subject is too small, detection might not work. No control wheel needed for any of these switches so it is very fast. I think for BIFs I mostly used Wide area (L), but things happened so fast and I experimented a lot, also with Auto-area. I have not scrutinized the EXIF to see what was most successful. I also have a setting for focus handover active. I limited drive mode to 10 fps, and used HEF raw to not go crazy on disk space. The biggest challenge was to preset focus so that the bird would become visible enough for the Bird detection, and actually aiming so that this could happen. If AF-on was pressed to early, focus go way off and one would have to repeat the preset.