I might stand silently with my lens trained on a bird for 15 minutes. The instant I drop the lens to swat a mosquito, the bird will fly away. Or, I will see a bird and reach for my camera, and it will fly away before I have the camera in hand. Or, it will be facing left but unexpectedly decide to fly to the right.
I was focusing on this little blue heron for quite a while. Without warning, it flew off, making me snip an eighth of an inch off its bill and even less off its wing feathers.
The easiest way to catch a bird departure is to reduce the zoom so the bird fills less of the frame and there is a split second more time to react. But, the bird image will be smaller and less sharp when cropped.
Decisions . . .