NASHVILLE, Tenn. - Tens of thousands of purple martins have been swooping at dusk near North First Street in a pre-migration gathering that's so large it's visible on national weather radar.
The martins swirl in a dark, tornado-like grouping each morning before spreading to look for food.
The sight has drawn a growing number of bird lovers.
"Oh my gosh, it's raining birds," said bird specialist Melinda Welton of Williamson County as she looked up.
The iridescent blue and purplish birds, the largest member of the swallow family, have for an unknown reason chosen the urban spot near busy Interstate 24 to gather before leaving to winter in Brazil.
The first sign of the birds Monday evening had been black specks dotting the distant Nashville skyline like pepper strewn liberally above The Pinnacle and other high rises.
For more than 15 minutes they sailed around, advancing as the sun dropped lower until masses were spiraling above a closed truck stop near First and Main streets in East Nashville.
Scott Sommershoe, Tennessee Wildlife Resources Agency ornithologist, who discovered the roost while cycling, estimated the number at 48,000.
"Probably they've been using the area for several years," he said.
The martin has a unique association with people, according to the Purple Martin Conservation Association in Pennsylvania. The birds, going back to Native American times, have tolerated humans well.
"They no longer nest in a normal wild situation historically that would have been holes or cavities in trees," said John Tautin, association head.
"Today, basically east of the Rockies they depend entirely on people to provide housing for them."
Gourds are hung and multi-roomed, martin "condos" erected in yards by martin-lovers, often referred to as "martin landlords." Birds come back yearly to nest and raise young.
The landlords are accustomed to the martins' sudden disappearance around Aug. 1, when the martins begin to gather into huge roosts such as the one in Nashville.
Before heading out, they spend four to five weeks as more arrive, foraying daily to surrounding counties to eat flying insects, including moths and June bugs, to fatten up for traveling.
About 11 million are estimated today in this continent, with numbers dropping in some locations, such as New England and the Great Lakes area, Tautin said. Tennessee is among states with a healthy population, with pre-migratory roosts near Dale Hollow Lake and elsewhere.
Monday evening, Steve and Cyndi Routledge of Clarksville, the first of more than about two dozen birders to arrive for the spectacle, watched a large bird perched on a phone pole.
"Words out in Cooper's hawk land that there's a buffet," Steve Routledge said.
The hawk has been a regular, waiting on the scene, and would soon be munching on one of the would-be migrants.
The birds' numbers rose as they spun in a formation that is distinct on radar when the weather is clear, a large, colorful donut shape with a hole in the middle. Best seen on the screen in the morning, the birds show up as green or blue colors that spread out from the hole and vanish.
Sharing the sky at dusk with a half pearly moon, the dark birds created an Alfred Hitchcock-like affect as they flowed in a river of wings and bodies into bushes and trees.
"This is all kinds of awesome," said Chris Sloan, with Tennessee Ornithological Society.
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