New way to test bird AF

I have been testing a few lenses (long story involving a broken 75-300 I), and have stumbled on a way to repeatably compare bird AF: find a 4K video of BIF and expand it to full-screen, then stand across the room and have at it. This one with several blue heron and trees has been pretty good: www.youtube.com/watch?v=dvPuEa3GAtE

Excellent idea, thanks!