I was standing with my back to the Snowy Owl which was perched on a tall pole. It unexpectedly swooped down right behind me. Only had time to turn around and try to grab a shot. The Ai-Servo did a very good job of catching onto this moving subject on short notice. This is a full-frame shot (not cropped). In 6fps mode I only managed 3 frames. The image above was the first of the 3 frames.

This shows two things. The 40D Ai-Servo is pretty good. Second, even on very short notice you can grab a shot like this. Snowy Owls can be slow moving in flight and aren't hard to photograph.