yeah your right Northern I'm shooting 5.6 at 400mm most of the time and probably would prefer a bit more reach. Sometimes I feel like I'm intruding on the owl's personal space, even though it ignores me most of the time, lol.
Steve thanks for the comment, I will probably decide between the 600 f4 and the 800 pf since a lot of the places I go are limited access due to conservation restrictions.
As for the lenses I see the 800 pf as a very good hand holdable, long reach, mobility option that seems to do fairly well in bad light conditions. The 600 f4 GM is faster and provides a way for me to stay out longer in lower light, plus having the versatility of being able to use teleconverters for added length if needed. Of course, putting a teleconverter on a 600mm f4 in the field when its not on a tripod could be a challenge for me, lol. That being said, both options are pretty similar in price because I would need to purchase the Z9 as well. I realize Nikon's backlog, and long wait times are a drawback to the Z system, but I do have the A1 with the 100-400 as a back up, so I am not without anything to shoot with.
Palmor I was most likely shooting in better light than you when I first arrived on location. As the sun goes down the owl's get more active and that's the lens I'm looking for. That 30 minutes before the sun goes down lens. That compromise of mobility, flexibility and low light performance when the sun is going down.
Thanks for the comments and the help, much appreciated. The people on this forum have forgotten more about photography than I will ever know, lol.