Autumn bird migration is starting and weather radar often detects large flocks of birds as they start their day. A large group of birds was detected leaving Green Island on Green Bay to feed just before sunrise on August 11, 2010. This radar signature, known as a "roost ring," occurs as thousands of birds leave their roost and fly into the air. The roost ring expands until the birds begin foraging after dispersing.
Below is an animation of radar reflectivity from the NWS Green Bay Doppler radar. Green Island is the small crescent shaped island southeast of Marinette. The first radar picture is taken at 5:31 AM; the last one at 6:14 AM. Besides the roost ring, rain can also be seen moving into the area from the west. (Animated GIF image.)
A similar dramatic roost ring was documented on August 10, 2006.