I suspect that the movement is more than just a predatory response factor. It is also used as an indication that there is something to give attention to. The ea

I suspect that the movement is more than just a predatory response factor. It is also used as an indication that there is something to give attention to. The eagle may be able to see the motionless mouse if it focuses on it, but it doesn't know to focus on it unless it detects the motion to draw attention to that area of its visual field.