This photo shows the flight of a bird while it flew past me at a beach in Seattle. I've superimposed multiple images to show the path the bird took. This photo was rejected by Shutterstock with the reason "Focus--Subject is blurry, too soft, or out of focus when viewed at full resolution.". I've looked at the image at different zoom levels but haven't been able to spot any lack of focus. Please suggest what's wrong with this image so I can try to fix it and/or improve future photographs. Thanks!