you can't handhold the 800, absolutely cannot be done and get anything worthwhile
i use my 500 on a wimberly head with a gitzo 1325, and that's heavy enough without the 13 lbs of the 500, takes a lot of practice to get the breath control and technique down, but when you do, it's a real sharp lens, even with a 1.4 extender on it
i've never even tried to handhold this lens for more than 1 shot, i've tried a couple beanbag/door edge truck shots, i too am hoping for some osprey shots this year, 2 new nests, much closer to the road, in town this year, plus my resident red tail pair have started a new nest out on the edge of our woodlot, do not know why they discarded their long time nest which was 50 feet in further and no clear shot with summer growth