For manual focusing, always remember how your lens focuses when you turn the focus right left and right. You should be quickly be able to realise if your focus is ahead of the animal or behind it. These days the DPAF of Canon is very good. When the animal is partially hidden behind vegetation, if you find the face tracking not working, then just use manual focus. I would always use manual focus in such situations, as the focus may hunt in the wrong moment and ruin the shot.

If you estimate the animal to appear at a particular place, then it is better to prefocus on that place, so when the animal appears near the place you had estimated, it would quickly come in focus.