The D850 +grip with the 200-500 is a bit heavy however you sling it on your shoulders . So is there an alternative . I never had any success with M4/3 though I did only have an M5 Mk1 with 16 mp. But the Panasonic G9 has 20MP so I looked at the pixel density v the D850. The G9 has 70% higher pixel density than the D850. Now if you take a 300mm lens the image size of a small bird it puts on any size sensor is the same so the G9 wins, more POI. However the longest lens for the M4/3 is the 100-400 but you can put a 600mm on the Nikon which boost the image size so I think you are about equal G9v D850.
But I find not all pixels are equal not even Nikon ones .The same lens on a D7200 gives a worse image than the same lens on a D850. So how good are Panasonic pixels or Panasonic "sauce" ?
Then you have the pose factor ...The guy in the hide with a D5300 and a 70-300 always give you a strange look when you have a D850/500mm, and you look at his kit and think "oh no not for me"
So will the G9/100-400 cut it ? or do you have an alternative?