AF.
As Shirley Bassey once sang,
Where Do I Begin 
Was at the beach house and there were few birds there, so naturally grabbed the camera and tried shooting them (with 100-400). Boy, was that a disappointing experience.
Attempt No.1: All focusing point, AI Servo, and Continuous -- No go. The cam keeps on focusing at the sea as if to tick me. Changed angle, tried peripheral focusing points. Up down, sideways. Still focusing at the sea.
Attempt No.2: Not to be outdone, I remembered C.FnIII-4-1 (Continuous AF track priority). So changed it to "focus point priority". That will do the trick, so I thought. Still no go.
Attempt No.3: The old fashion way. Single and middle point. That's a go! And most successful.
There were many combination I tried in between but don't want to bore you more than I already did
Any advice will be appreciated.
Cheers