Sorry bro, no prior experience with Mk3 so far, this coming Sunday will be the first time. :sweat: But have used Mk2 before for BIF, I think BIF is one of the toughest things to test the AI servo focus of the camera with, I don't wanna go into detail, but generally most Canon cams including 1 series track a moving bird against blue skies pretty well, but against stuff like water or greenery the focus will sometimes get thrown off momentarily (i.e. focus on bg). This is using central AF pt with the AF pt expansion. So far have not tried BIF using the "Ring of Fire" yet. But generally speaking, 1 series bodies do track BIF much better than their xxD or xxxD counterparts.