Bird in flight technique question
I was out shooting waterfowl today. With my 7d 2 and 100-400 2, I was getting very good, sharp images as they came into land. However, of the six or seven images taken per bird coming in, several were out of focus ( like 2 out of the seven). So my first 3 are fine, my next two out of focus and my last two in focus. I bump my focus as they come in, probably three or four times. When the duck gets low on the background, I end up focusing on the water and not the bird. Shaking due to my age probably has something to do with it (as well as being on one knee to get low). Here's my question, do I need to bump focus so much or will the autofocus system hold on the duck when it is coming in?
I'm using bbf, centre point focus, al servo, etc. ( following Art Morris' guide).
Thanks for the help.