Accel./decel. tracking
Since birds can change directions quickly their relative speed towards/from your camera changes quickly as well, even if they don’t accelerate/decelerate from their point of view. I recommend configuring the camera to a accel./decel. tracking value of +2 which means that the camera will not hesitate on giving an immediate response on every measured misfocus, a behavior that could be considered as being a bit “nervous” but is in my opinion inevitable for fast and inconsistent moving subjects.