It's a gull flapping its wings. Being white underneath and grey on top, as its wings come down it blends in with the background, giving the 'flashing' effect…..not an alien spacecraft, but nice fireworks though!
Living in Chicago, I know there were no fireworks this year on the 4th at Navy Pier. City canceled them due to low funds. This is old footage posing as current which leads me to believe the object isn't a UFO either. (There were however fireworks on the 3rd, navy pier puts on a small show on its own every Saturday night.)