I think it is not necessarily a 600 mm prime lens, which is very heavy to use for documenting purposes. There are 150-600 mm lenses, they are much lighter and good enough for documenting purposes, even for bird photography. New generation lenses like Canon RF 800 mm F 11, are also much lighter (only 1260 g) compared to the canon premium EF L lenses. So the camera setup would not exceed the weight of a spotting scope setup. Other than that, the weight of the setup wouldn’t have much effect if you are birding from a hide.Thanks for the correct info on magnification.
"Most birder here use focal lengths of 400-600mm to document observations. Some use even longer focal lengths."
Whew! I've used Canon 600 mm f/4 IS and ISII lenses for 14 years. I don't know of any sane person on earth that carries a 600 mm lens while birding... it's absolute insanity due to its size and weight and the absolute need for a heavy tripod and a gimbal. Good luck in the woods carrying a 600 mm lens and a tripod/gimbal looking for birds -- or even in open country in anything but hospitable terrain. Like carrying an anvil.
Even the latest, lightest, 6th generation Canon RF 600 mm ($13,000 USD) is way too cumbersome to be practical for birding. Great for photography, but that's not birding, it's photography. You don't carry a 600 mm lens up to Hawk Mt. (HM) to count raptors -- you use binoculars. And even the much lighter/superb Canon EF 400mm f/4 DO IS II-- a lens I own --would be a handful to carry up to the HM North Lookout.