Hey Bear, while i haven't gone D850 -> Z7, I did try going D750 -> Z6, so I do feel i have some insight that would have meaning.
There were two things the Z brought that were improvements. IBIS and, surprisingly for me, the EVF. It's much easier to see what you are doing in low light and zooming in is incredible for manual focus. Those were the only real benefits I noticed after going D750 -> Z6 after a month of solid use. To be fair, those improvements were absolutely huge in the right circumstances. Video is far better, if that matters.
Downsides are reduced Dynamic Range due to banding and inferior body control. I have heard Z6/Z7 AF-C isn't quite up to par with the DSLRs as well, but I don't have any AF lenses.
As it seems that you focus on birding in daylight, I'd be hard pressed to see advantages in the Z7 over the D850 besides wider AF coverage in your case, in all honesty. I'm guessing you already use VR lenses, so I'm not sure how much benefit you'd gain from IBIS, and I can't imagine the EVF being better for daytime birding. While zooming in for focus is effective, it is certainly too sluggish for use in shooting wildlife. The current Zs are slower cameras in operation than the DSLRs are in general. If I shared your taste in subjects, I'd be waiting for a Z body to come long with proper controls and a non-sluggish EVF at a minimum.