Meant to add. I go to Australia Zoo quite often as my grandaughter works there and I like to see her entertaining during the croc and bird show and it gives me a chance to shoot wild native birds in the bushes there.
It's not a focus issue but I take the 5D3 with me and the 100-400mkll as opposed to the 7dmkll as there's lots of dense undergrowth where the wild birds lurk and I need to up the iso to shoot them. The only downside that I find to the 7dmkll is, (and it may just be my copy or user error), that the noise in the shadows is far, far worse than the original 7D. I shoot one stop to the right and still have lots of non-keepers due to noise and fringing, even at only 400 iso. Again, it may be error on my part being new to bird/wildlife shooting in mainly dappled light, and perhaps I should be 2 stops to the right?