Thanks. With the sea turtles I don't worry much about slow focus, they don't move that fast. I usually focus on the bodies in the water using BBF, then wait for the head to pop up. Pushing the shutter quickly when they pop up is the trick since the head is often only up for a second or two. I did borrow my wife's 200-500mm and used it with a 1.4x and it was fine for a red footed booby in a nest with young, but it was broad daylight, and again the booby wasn't moving and it was bright. In low light it might be a different matter.