Dan
I was using the 800, not the 300 or 400 or 500 for this image -- the birds would not let one get that close.
An 800mm+1.4 has a lot less DOF than a your 400+1.4x....
At 1120mm, at 15ft, DOF is 0.01 ft at f8.
With a 400+1.4x(500) you have 0.09 ft DOF at f8 (9x more DOF).
For same distance...however image size on same sized sensor is >4x for 1120 vs 520, so you'd have to get closer to be equivalent. That would also make a difference.
Bottom line - its what was needed for the image for the lens choice.
You might get a copy of DOFmaster and give it a try for comparisons
Hope this helps
Dan
Have you signed up for Artie's visit in May?
He's got a couple of classes that would be good for anyone making bird images. You'll likely learn a lot. He's an excellent speaker. http://tualatinriverbirdfestival.org/events/ Right down on one of both of our favorite local refuges.
Don