How does the new Eye-AF compare for ease / speed of use, and accuracy compared to the methods folks started out shooting Nikon Z Mirrorless (such as Eric mentioned above for static birds). What about BIF ?
Judging by size of the "eye detect" focus square I wonder whether this feature has any relevance for bird photographers at the less than ideal distances most of our subjects usually are ...... ?
I have to say that looking at this very interesting YouTube "test" I'm a bit disappointed (and surprised) that Nikon isn't further ahead of the curve in Mirrorless AF given their vast experience at Pro photography ..... Sony seems well ahead in Eye-AF .....
I just got back from a week at Cape May with more than 4500 bird photos using the Nikon Z6 and the new firmware update. AF is excellent for birds in flight as well as perched subjects. I'd put it equal or better than the D850.
My primary AF mode for birds in flight are the Wide modes - Small as a first choice and Large if needed. Prior to the firmware update, the camera did not use closest subject priority in those modes as it should. After the update I did not have a single frame where the camera picked up the background over the foreground. This was especially useful for flocks but it is why I use the Wide modes for BIF.
It's always a challenge to track a fast subject. I have never used the Auto tracking modes like 3D. But Wide modes are very effective and I could not blame the camera for any issues I had in following small, fast subjects. Focus stayed on subjects and quickly returned if I lost focus temporarily.
I did use Dynamic AF some fo the time - mainly when I had a foreground obstruction and wanted to be more precise than Wide modes would allow. Dynamic did the job as expected, but is harder to use for rapid movement. It's better for slow movement - a subject walking or hopping thorugh a thicket or something similar.
None of this involves Eye tracking. That's a different function and not something I would use for bird photography.