Action photos require your EOS camera to track a subject and continually update focus until the photo is captured. Selecting Servo AF is only the most basic step to make this happen. Many cameras have a range of additional settings to refine how a camera autofocus responds to different subject dynamics.
Almost regardless of the actual subject speed, it’s how it moves that determines how AF needs to respond; a fast car on a race track moving towards or away from the camera is consistent and quite easy for the camera to predict where it will be when the photo is taken. A small bird in flight changes direction much more rapidly and so benefits from different focus settings to maximize the chance of each shot being sharp.
