Here are my birding / nature AF settings, in case anyone is interested:
basic AF settings: AF-C, custom setting 6, shutter priority: release
- tracking sensitivity (0 - 4): usually 1 or 2, depending on what I'm photographing. If the trajectory is unpredictable or hard to track, I'll choose 1; for more predictable objects (e.g. birds like geese or aeroplanes), 2.
- speed tracking (0 - 2): 2
- zone switching: either auto or front; auto is good if you're trying to pick out a target amongst other complicated objects both in front of and behind the subject, such a bird in reeds or a butterfly visiting flowers. If I know I'll always want the camera to focus on the foreground, I'll choose front.
For static subjects, I use single point AF with the box set to the second smallest setting. For moving subjects, if I'm really on my game I'll use the largest box size or the 3x3 zone setting. Occasionally I'll use the 5x5 box if the subjects are very unpredictable or fast and/or the background has little in it that will compete for focus.
I find the XF100-400 is best stopped down to f/7.1 or f/8.0 when at 400mm; although I have the 1.4x TC, I don't find the photos for it really sharp unless it's stopped down to f11, which makes AF more difficult.
There are a few shots
here that I posted a little while ago, and I've attached a few shots from a recent trip to the Farne Islands.