I own a Z6 since December and decided to take it to its first real challenge in terms of autofocus speed and tracking with my Sigma 500mm F4 Sport lens on European bee-eaters
Last year I had the 400mm F2.8 G with the D500 and I was amazed by the AF performance (almost as much as by the wonderful colors of these birds). I found that group AF worked best for me in this situation, the camera would sometimes lock on the background but not so often while still tracking if the subject went on the corners of the frame.
The Z6 doesn't have an equivalent for group AF, I struggled trying Dynamic Area / Wide S and Wide L but it would not work, always locking on the background.
Keep in mind the birds are not so large (less than 30cm) and the average shooting distance was ~15-20 meters, they are not easy to keep in the frame with the way they fly.
So I tried auto AF and it just worked...
The trick I used was using the memory recall function of the lens set around 10m, if the AF got lost on the background I could get back almost instantly and try again, the lens would go again and find the first interesting thing to focus on.
The EVF giving a real DOF preview of the image, I found the viewfinder much clearer to see the subject when shooting than with my DSLR (the D850 now).
Using "back button AF", the auto AF function of the Z6 would keep following the bird quite nicely in all directions.
I kept the lens wide open at F4, shutter speed at 1/2000s or 1/2500s and auto ISO limited to 200 to avoid over exposing the whites when the bird would fly in front of "dark" trees. (yes I know locking at 100 ISO would have given the same results after post processing)
They are all quite heavily cropped, the Z6 sensor can capture a lot of fine details as long as the lens is sharp, and the 500mm F4 Sigma Sport is sharp.
AF Accuracy is just better, that allowed a few very nice images taken with the 1.4 Teleconverter at 700mm F5.6 on static subjects.
That's why I am still hesitating to replace my D850 by a Z7...
A few other images, and if you are curious and want to a little see more of what I do check my flickr gallery: https://www.flickr.com/photos/96833549@N08/