I've found with the 400mm f5.6 that the hardest part about getting BIF shots is finding the bird in the viewfinder. It's like looking through the cardboard tube from a roll of paper towels; the FOV is tiny. So I've been experimenting with tracking with both eyes open. With the 5D MkIII in expanded point mode, I get a circle in the center of the VF. With both eyes open, that circle appears to be floating way out in front of the camera. Since I have both eyes open, I can see the bird and move the circle onto it, and then the bird appears in the VF. I close my left eye and focus/shoot as normal. It took a little practice, but I've become quite proficient at it. Maybe everyone does this and I'm just waking up to the idea?