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Bird roost on radar this morning!

Bird roost on radar this morning!

by Ben Pine

WHAS11.com

Posted on July 3, 2012 at 6:51 AM

Updated Tuesday, Jul 3 at 7:30 AM

What you are looking at are flocks of birds taking off right at sunrise this morning, detected with Stormtracker 3D Doppler Radar. 

This is most likely a large roost of Purple Martins, which will gather in the 100s of thousands, and take flight all together to find their first meal of the day.  They will all return around sunset.  This happens across the eastern half of the United States this time of year - it's one leg of their large migration pattern from South America. Below is a map of the Purple Martin migratory range from the Purple Martin Conservation Association.

The National Weather Service did a story about the Purple Martin on radar as well, take a look at the cool radar loops.  Here's their story below...

"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.
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."

http://www4.uwsp.edu/cnr/faculty/russell/images/purple%20martins.pdf

http://www4.uwsp.edu/cnr/faculty/russell/images/wsr-88d.pdf

 

 

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