Radar that's For the Birds!

Posted Updated
Radar Reflectivity 6:31 am 25 July 07
Mike Moss

A couple of times in the past few weeks, we've noticed a neat feature on weather radar displays around the area that occurs only in certain parts of the year and at certain times of the day. If you expand and look closely at the image above, you'll see three annular circles of radar reflectivity, the closest to our area being centered near La Grange in western Lenoir County, with the ring at the time of the image just east of Kinston in the east and just east of Goldsboro on the western edge. You'll see similar circular echoes over southeastern Brunswick County just west of the Cape Fear river and Carolina Beach, and a third over northern South Carolina not far southwest of Rockingham in Richmond County.

These are not a meteorological phenomenon, but a biological one, sometimes called "Roost Rings." These occur when large concentrations of birds (in particular, at this time of year, Purple Martins) staging for migration disperse from  their roosting sites around sunrise to go out foraging for food. Sunrise on this day was 6:17 am and the rings were still expanding at 6:31 am when this radar image was captured. The rings eventually disappear some 45-60 minutes after the initial ring developes, as the birds become less concentrated and descend to feed.

These "roost rings" are not at all the only times you'll see biological returns on our radar displays, whether from Doppler 5000 or from the network of NWS NEXRAD radars in the region. Many radar echoes can be traced back to nighttime bird migrations, flying insects feeding, bats dispersing to feed around dussk, and so on. You can read an excellent primer on this entire subject, nicely illustrated with sample radar images, at the following site from Clemson University (just click the "next" link until you reach the descriptions of insect, bird and bat returns):

Also, if you live in the area of any of the three "Roost Rings" in the image above, and you're familiar with the roost areas there, post a comment below and let us know what you see from the ground, and if we're on target with the assumption that we're looking at Purple Martins!

UPDATE - after looking into this a bit more, I've learned that the Martins prefer to roost in areas near large water bodies or marshy areas with reedy islands, and that roosts can contain an average of 25,000 to 100,000 members. The three circles on the image above are located near rivers (The Cape Fear in Brunswick County, the Pee Dee in South Carolina, and the Neuse in Lenoir County). A scan of satellite and aerial photo imagery along the Neuse between La Grange and Kinston seems to indicate ideal habitat there, as the Neuse meanders sharply through that area and there are numerous cut-offs that have created small islands and "oxbow" lakes. There is another well-known roost in northeastern NC near Mann's Harbor, in which the Martins use the western end of the William B. Umstead bridge. These roosts begin forming in late June, peak through July and August and dissipate in September as the Martins depart on their migratory Journey to South America, especially Brazil.


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