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Roosting Birds Detected on NWS Doppler Radar

Just before sunrise on numerous mornings since mid-July, a curious doughnut pattern has appeared over several locations on NWS Wilmington, OH Doppler radar imagery. This feature, 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 observed roost rings in recent weeks have most likely been caused by purple martins, which congregate in enormous colonies between mid-July and early August, once their fledging period has ended. By late August or early September, the martins begin their migration south, and then the roosting activity of other bird species nears its peak.

The unique doughnut pattern of these roost rings is the result of the martins 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. Purple martins 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. Take a look at the two NWS Wilmington, OH radar animations below, taken right around sunrise on August 2nd and 3rd, 2010, and notice how the rings appear in almost the same exact locations both mornings.

Image 1: An eight-image animation of base reflectivity, starting at 6:10 AM and ending at 7:17 AM, August 2.

Image 2: An eight-image animation of base reflectivity, starting at 6:22 AM and ending at 7:21 AM,
August 3. Rain moving across the northern portion of the area obscured some of the usual roost rings there.

With no rain across the entire Ohio Valley, the early morning hours of August 2nd provided an excellent opportunity to observe roost rings popping up all over the region. In the regional radar animation below, notice how the rings appear from east to west in conjunction with the timing of sunrise.

Image 3: An eight-image animation of base reflectivity, starting at 6:12 AM and ending at
7:36 AM, August 2.

Why do the rings only appear on radar during the birds' morning departure and not during their evening return also? Atmospheric conditions have a big impact on the path that the radar beam travels. On a typical early morning, the beam is bent slightly downward due to an inversion in the atmosphere, and it detects objects closer to the surface more easily. This is not usually the case during the evening, when an inversion is not typically present. Additionally, researchers have found that purple martins return to their roosting sites at lower altitudes than their morning departure, and thus they normally avoid detection by the radar beam in the evening.

For more detailed information on using NWS Doppler radar to monitor the roosting habits of purple martins, check out the two links below.

Use of Weather Radar to Characterize Movements of Roosting Purple Martins
Large-Scale Mapping of Purple Martin Pre-Migratory Roosts Using WSR-88D Weather Surveillance Radar

Michael Kurz, Meteorologist Intern

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Wilmington Ohio Weather Forecast Office
1901 South State Route 134
Wilmington, OH 45177
Tel: (937) 383-0031
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Page last modified: August 12, 2010.
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