I agree with what`s been said about full frame being cleaner and a good deal better than crop sensors generally . But in my opinion you have to be that much closer , once you start cropping the full frame starts to lose advantages I would think , I got a 7Dmkll 2nd hand last year I think it was , and I am very impressed with it , as birds are your subject I would go with the 7D mkll , you are seldom ever close enough and will likely still end up cropping even on a crop sensor body . You can see a world of difference from full frame but you gotta get close enough , and of course they do have an advantage of better hi iso performance , but I think the 7Dmkll is o.k up to iso 1600 , and find the Af is excellent for a Camera of it`s time .