A few selected examples of some African birds demonstrating how a Mirrorless AF system "frees up" framing the scene to capture the Gesture in the image (ie the behaviour in the moment). These images are more a proof of principle in testing: as part and parcel of refining setup of the camera and lens(es). The major differences IME are the wider canvas of search space for the AF cursor, and the Subject / Eye-AF modes. And all the better if the Area-Mode is constrained to W-Area or S-Area (roll on Nikon with the awaited Custom Area modes as in the D6!).

After many years progressing with Autofous on the F90x through a sequence of DSLRs, I have found the Z9 sure makes life a lot easier. From 2016, these included D500, D850 and D5, and also trying the Nikon Z7, which also has many of the well known MILC AF advantages but it too often grabbed the background. (the Z9 does this too but it's quicker to rectify on the fly).

Far more seriously, to the core the Z9 is a thoroughbred Nikon camera. This means Custom settings are essential to leverage performance, and it is vital to setup choices in the AF area modes to allow very fast switches as challenges happen. Back on Z6 and Z7, as I won't stop repeating, Nikon would only improve the AF effectiveness of the Z7 to use at least 3 AF modes, I would still be using my Z7.

Sure one can keep the single-point of a DSLR on the eye but this can get tricky to frame and track and all while shooting, especially with a small bird turning its head. I often use Single-Point / S-Area on Fn1 and Fn2 respectively, with W-Area on BBF (Ln-Fn is set to 3D-tracking, both MultiSelector buttons to AutoAF): and yes, subject-detection is enabled.
And time and again, I've found so many subjects find the much quieter mirrorless camera far less intrusive. I have the Z9 shutter set to Volume=1, Pitch=High, and it's quick to set Silent_shutter via the i-Menu

juvenile Klaas's Cuckoo yawning, Z9, 500 PF + TC14 III


Klaas's cuckoo stretching: the oversized hallux stands out: the position of the big toe is reversed during the embryonic development of birds, and enlarged in some clades of perching birds


juvenile Klaas's Cuckoo preening, Z9, 500 PF + TC14 III

A pair of Hadeda Ibis visit my front lawn often to probe for earthworms and other invertebrates. This is when not only 15fps but 20fps helps to grab the 'moment' as the prey is processed with the bill very fast, and mostly using 3D Tracking in W-Area. Confining the AF cursor within the rectangular frame is not absolute, but this focal region acts as a strange attractor, and helps to avoid the camera grabbing another bird or something it might mistake for a head or eye. Of course, the key advantage the AIF based autofocus takes care of a sharp eye, as the photographer is concentrating on the bird's feeding and shooting short bursts

All images: Nikon Z9, 180-400 f4E TC14 wide open (either f4 or f5.6) 1/2500