Roost Ring: Donut Image On Weather Radar Formed By Birds

By John Dodge

CHICAGO (CBS) -- This morning, just south of Cedar Lake, Ind., an unusual donut-shaped image popped up on the weather radar.

There was no cause for a weather alert from the National Weather Service for this somewhat unique phenomenon.

It is known as a "roost ring," and it was detected along the Kankakee River in Northwest Indiana on Tuesday around 5 a.m.

According to the National Weather Service, a roost ring is a detection of birds as they leave their morning roosting site.

The radar cannot detect the species, but this particular ring could be Purple Martins, which gather in groups before the late summer/early fall migration.

The roost ring pattern is the result of the martins departing their roosting site in all directions. As they travel farther and reach higher altitudes, they are detected by radar.

In the coming weeks, the NWS expects to see more roost rings from a variety of birds.

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.