August 10, 2006 Bird Roost Ring on Radar

NWS Doppler Radar Detects Birds Leaving Green Island

A large group of birds was detected leaving Green Island on Green Bay to feed just before sunrise on August 10, 2006. This radar signature, known as a "roost ring," occurs when the radar beam detects thousands of birds simultaneously taking off from their roosting site around dawn to forage for insects.

Leading up to fall migration, a number of bird species are known to gather at large communal roosting sites, which are often detected by NWS Doppler radar. The unique doughnut pattern of these roost rings is the result of birds departing their roosting site in all directions, roughly in equal densities. As they travel further from their roosting site and reach higher altitudes, they are detected by radar until they either rise above or drop below the radar beam. Some species typically return to the same roosting sites in the evening, which are usually situated near bodies of water. As a result, the roost rings are often observed in the same locations on radar over the course of several mornings.

Below is an eight-image animation of radar reflectivity from the NWS Green Bay Doppler radar. The first radar picture is taken at 5:25 AM; the last one about 6:15 AM.

Radar animation

Another example of a roost ring from Green Island was documented in August 2010. Click here for more on that event.