Lessons learned on my first BIF outing:
1. You were spot on, Dave. ISO 800. You really need ~+2EV or you get nice bird silhouettes.
2. You miss lots of good shots if you spend time chimping.
3. The 6D's buffer *sucks* compared to the 7D's. I think "6D" comes from the number of times I said DAMN! while waiting for the buffer to clear.
4. The 6D's AI Servo seems plenty accurate for me. It even held focus through branches for a short distance. (See below.)
5. The 400mm f5.6 is one sharp lens. Check out the 100% crop below.
Group Flight
Solos
100% Crop from image above.
100% crop from buzzard behind the trees.