The NWS Doppler radar near Sullivan in the southeast part of Wisconsin detected several flocks of birds leaving their roosts just before sunrise on November 15, 2009. As the birds take flight in the morning, they often leave the ground in what is called a "roost ring"--a radar signature formed as they fly into the radar's coverage area.
In the animation below, you can see at least three roost rings around the radar (denoted by the red dot and "KMKX" designation in the center of the image), including a large flock of birds which appears to leave the Horicon Marsh area of Dodge County northeast of Juneau. That flock of birds flies directly over the radar as the birds move south. The birds were flying at over 30 mph, moving along briskly by northerly winds.
Radar animation starts at 6:32 AM and ends at 7:42 AM. Images may take a few moments to load.
For another example of a roost ring, click here.