I do own one lens that no longer autofocuses on the Z but I don't use it much anyway (my older 85mm). I could use it fairly easily using manual focus though becaues the focus peaking makes manual focus pretty easy. My lensbabies are all manual focus as well, regardless of what body I'm on.
I personally don't shoot BIF, but I have honestly never used any of the tracking focus modes anyway. If I were going to shoot a bird, I would choose a single point and track the movement with my entire camera. This is how I shoot my kids swimming and my son playing lacrosse. I get a fair enough share of keepers tracking with my own eye and arms.
I get that there is a mental thing with switching from F mount to Z mount. I switched because I wanted a wider area of focal points and I liked the smaller body. The array of FPs is enough to keep me firmly in the Z category and honestly I find focus faster with my Z than my D800/D700. The adapter is truly a non-issue and I thought it would be a big deal and it gave me a lot of pause.
I'd actually recommend you rent one (with the adapter) and see how you like it. It would be far less expensive to switch and use the adapter than to switch platforms. I've already done that once in my life and don't want to if I don't have to.
I know there are people like AFB who have zero interest in mirrorless, and I understand that. I had zero interest as well until I started seeing some images from the Z series and started to understand more what it could do that a traditional SLR could not. But I would never push someone from SLR to mirrorless, especially if you already like your gear.
However, if you ARE considering mirrorless and also already shoot Nikon, I do think that not even considering the Z is foolish. One may still end up with Sony but the Nikon is seriously underrated, and many people are still reading reviews from when it first came out, not taking into account the upgraded firmware that put it much closer to Sony. I've nver held a Sony but a lot of people hate their menu driven system, and the Z for me is so much more intuitive. I have never used a camera body that I had every buttom memorized, and I can honestly shoot and change settings without taking the camera away from my eye. I have never been able to do that, even with six years with the D800.
Some us of even try for that. 
