Put me down in the heron test in the camp of "full framed cropped 5d3 equals or exceeds 7d for bird photography". My test was similar to the herons except I used kingfishers in extreme range. The 5d3 images yielded better detail when cropped to equal framing. My settings for birds in flight are usually shutter 1250-1600, f 7.1 to 10 and ISo on automatic. With the 7d Letting the iso go above 320 in autoiso has its own issues.
So for me a ff has more sharpness than crop given the same lens and distance to target.
AE-1, T90,EOS Elan 7e, EOS-M, 7D, 5D3, 1dx, ef40 2.8, ef 85 1.2ii, ef100 is 2.8 macro, ef300 2.8ii, ef400 2.8ii, ef600 is ii usm, ef8-15 f4, ef16-35 ii, ef24-70 ii, ef 24-105, ef 70-200, 2xext ii, 2xext iii, 1.4x ext iii.