Thanks Lester, appreciate the your comment on the Sanderling head-on shot.
I used Servo, Whole Area, Subject tracking, Animals, Eye Detect, Switching Tracked Subjects=1, Case A (interesting, I thought I had it set on Case 2), Electronic Shutter., H Continuous.
In this instance, the backgound was fairly clear, and the little guy though coming fast was quite a way off initially. I had time to made sure focus was locked with shutter half-press before fully pressing the shutter to start the burst. 3 of 5 shots in the burst were in focus. I tried a second burst as he got closer, but I must have lost focus lock because they were all garbage!
Thanks that is interesting.