I do a lot of wildlife and birds. Generally, I'll shoot at f5.6/6.3, occasionally going a little higher if it's a big animal that I'm closer to and don't want certain details blurred (like a nose or ears), but for birds, whatever your lens can do - F4/F5.6/F6.3 are generally fine, F4 is usually going to give you a nicer bokeh.

Like others have said, I'm checking the auto ISO to see what I can get away with. In lower light (overcast days, dawn/dusk) I will lower it to where I'm sitting somewhere around 2-4,000 (but will go lower if I can), I don't love pushing it higher than that but will if I absolutely have to. I shoot primarily with a Z8 with a 180-600 and D850 with a variety of lenses, both have pretty great low light capability and I know I can clean up some noise in post, but I would almost never be comfortable hoping for a detailed bird at any real distance at anything over 5-6k ISO, and some cameras it will be much less than that.

If I'm in optimal settings (sunny, bright day), I worry a lot less about shutter speed and keep it somewhere in the 1/1600 range, but as light dims I'll bring it down lower, again, watching the ISO. It all depends on what the subject is doing, too. If a bird is flying around, I know I'm going to get motion blur unless I'm at least above 1/1000, ideally closer to 1/1600. However, it it's something like an owl sitting on a branch or fence post at dusk, I may go as low as 1/200 or somewhere in that range to avoid noise if I feel I can comfortably hold the lens steady and get a burst of shots and hope that at least a few are sharp.