Cousin David titled this ‘motion blur by luck.’ I would call it panning the camera. He shot the image and moved the camera in the same direction with the geese in flight. This will blur the background while keeping the subject in focus. It’s another way to make ‘bokeh’ without playing with depth of field. This is especially helpful in sports where you can visualize a race car going by and the crowded background is a pleasant blur. It enhances the notion of action. With birds this can be difficult because often the flight is less predictable. Here David got a fine shot. I didn’t notice the goose on the right. It was obscured by the background. But the goose on the left is nicely isolated.