I posted two images, one from a 1DX MK II with a Tamron 150-600 G2 zoom and one from an Olympus OM M1X with a Panny LeicaDG 100-400 - both shot in pretty bright light, at fairly stationary targets handheld, so folks can begin to compare the quality offered by each system.
We were travelling in a snow coach in Hayden Valley in Yellowstone, looking for a fox, when what did our driver, Doug, see, but this delightful red fox walking along a ridge line several hundred yards away... With a little patience it gradually approached the road, sat down, and observed the passel of photographers in its path ( I would say a passel must be at least 60-80 or more ).
My fox image was shot while the fox was watching us, from about 30-40 yards away at 600mm f8 1/1250th at ISO 200. The snow varied in depth from 6-12 inches on the road to 3-6 feet deep off the road in the valley. Mostly it took some luck, some very nice light, and possession of a suitable camera/lens combo for wildlife.
I have much better lenses than my Tamron zoom, made by Canon, but my Tammy is so handy, relatively light and fast to focus, that it is far away my most often used and successful wildlife tool for handheld shooting.
Snow always makes such a lovely diffuse reflected fill light - I knew that if my focus was precise, I would capture several nice frames - I actually have a great many - I also got it jumping up and diving into the snow, but not nearly as close. I have even more frames of coyotes, but my wolves were all very very far away this year.