Z7: It is not for action. It will take great portraits, but the tracking and AF is not up to moving birds. It also has a noticeably slower power-up from standby time that causes me to miss shots. It is not instant like the d500 or d850. And while its powering up, the viewfinder is black, unlike the DSLRs. It can take wonderful shots, especially with a 1.4x TC, because it focuses on the sensor not through a prism, so you don't need to futz with AF Fine-tune like on a DSLR. I'd wait for the next version, or maybe a software update to help the AF tracking.
Marc
I have the Z6, D850, D500 and D7200. I have more than 1500 bird photographs with the Z6 including plenty of bird in flight images.
The Z6/Z7 will track relatively well, but you need to throw out the ideas that your settings for a D850 will work on a Z6/7. The viewfinder will power up just fine, but you need to use the camera in a different manner. The Z6/Z7 does use Phase Detect AF for all AF-C area modes, and focus is relatively fast, but you still may need AF fine tuning either to shift your desired focus or to adjust for lens and mount specific issues that affect Phase Detect focus.
As far as AF is concerned, the Dynamic AF of the Z6/7 is similar to the D500/D850 in that it requires you to quickly return the AF point to the subject. If you can't return the AF point to the subject, you need to use Wide AF mode. Wide is equivalent to Group on the D850/D500, where the camera uses a group of AF sensor as one sensor. There are two options - Wide Small and Wide Large - roughly the same as the way Group covers the sensor on the D850 and D500 respectively. With Wide or Group, the intent is to focus on the closest subject. That's a place where the Z6/Z7 does not do as well as the D500/D850 - especially with busy backgrounds. That issue needs to be corrected with software. I never use 3D tracking with any of these cameras for bird photography - it's got a different role such as for colorful subjects moving directly toward the camera.
The AF success rate or keeper rate for me on large birds in flight was around 75% with the Z6 compared to 90% with the D850 and 72% with the D810. With smaller birds that number gets much lower and is based on technique.
The EVF is fine at a high frame rate. If you have it set correctly, you have a slight strobe effect - similar to the shutter on a DSLR. It's different but I had no problem tracking a subject for 8-10 frames.
Shutter wake up is different. It takes about 0.8 seconds to activate the viewfinder. What I learned to do was touch the shutter when I saw a potential subject. By the time I could raise the camera to my eye, the camera was fully active with the EVF operating normally. When working a subject for long periods, I set my standby timer for a little longer, and wake the camera periodically.
The big advantage I have using a Z6/Z7 is with small birds. The smaller AF sensors allow better focus on small details or small subjects in wooded or shrubby areas. Tracking a small sparrow on the ground is much easier than with my D850. Dynamic AF does a remarkably good job of helping me stay on a hopping subject and avoiding focus on sticks and brush instead of the subject.
Another big advantage with small birds is the ability to zoom through the viewfinder. Because you have an EVF, you can zoom before or after making the image to view a magnified image. I program the Fn1 button to zoom, and set it to either 50% or 100% depending on the subject. In effect, the EVF works like electronic binoculars with a stabilized view. Using the Fn1 button to zoom, I can identify a small bird at a long distance and concurrently grab in image for review. I can also use the zoom to watch a subject and shoot with precise head position or a catchlight in the eye. On recent bird walks I was able to identify and photograph 1-2 additional subjects each time due to this capability.
My favorite lenses for bird photography are the Nikon 600 f/4 AFS VR, the 300 f/4 AFS, and the 200-500 - all with the FTZ. The first two work very nicely with the 1.4 teleconverter. The 300 f/4 is my walkaround birding lens using the TC14E II teleconverter. This combination takes an older lens without VR and has the benefit of IBIS. AF using the FTZ is the same as performance with the same lens on my D850. There is no real difference - and a possible edge to the lens with the Z camera. I find focus a little more accurate with the Z cameras - even using the FTZ and F-mount lenses.
My conclusion is for large birds in flight, the D850 and D500 are about 20% better than the Z6/Z7. But for slow moving, hopping, or perched subjects the Z6/7 is significantly better and easier to use.
I can take the D850 or Z6 out today and produce professional quality images consistently. The D850 is a superb all around camera, but the Z6/7 is also a very good camera and tops the D850 in a number of ways.
The D500 - like the D850 and D5 - has a separate AF processor. That makes the D500 and D850 faster to focus than other cameras like the Z6/7 and D750, D7200, D7500, etc.
Here are some samples with the Z6 and the FTZ.
The gray catbird is a 100% crop of the second image using the 300 f/4 and the 1.4 teleconverter.
The other two images are with the 600 f/4.
