Thanks again.
Alan, the 400 f5.6L is sharp throughout. Depending on some variables I shoot anywhere between 5.6 and 8.0 but am at 5.6 more than Im at 8.0 simply due to my desire to keep my SSs high, typically 1/1500 to 1/2000th. That particular day the sun was in and out every couple minutes and the birds were at times in cover so I left my metering mode in AV and f5.6 to ensure I kept my shutter speeds up for fear of losing a swoop chase or takedown to motion blur.
J