North Carolina drivers are commonly warned about deer and other animals crossing the road. But on one bridge over Croatan Sound, the birds are so thick this time of year that extraordinary measures are necessary to avoid them.
Purple martins, a type of swallow, roost on the I-beams under the deck of the William B. Umstead Bridge between Roanoke Island and Manns Harbor. From now through August, they leave the bridge at dawn after a night’s sleep then flock around it again at dusk, after feeding on flying insects in nearby marshes, fields and forests.
The birds used to die by the thousands, as cars and trucks barreled over the bridge at 55 mph, says Michael E. C. Gery, who lives on Roanoke Island and heads the Coastal Carolina Purple Martin Society.
That changed starting in 2007, when the society persuaded the N.C. Department of Transportation to put up warning signs and lights and reduce the speed limit to 20 mph when the birds are swarming.
“Since the presence of a 20-mph speed zone on the bridge, martin mortality has declined considerably,” Gery wrote in an email. “Our informal counts run fewer than 300 deaths during the roost’s active period.”
Many North Carolinians know purple martins by the large houses and clusters of gourds that people put up near their homes throughout Eastern North Carolina. The birds nest and raise their young in these houses each spring.
When the young are large enough to fly, purple martins begin making their way to roosts, where they build up their strength for the fall migration to South America. Gery says martin families from as far away as 150 miles come to the Umstead bridge, where more than 100,000 birds will gather before heading south.
The 20-mph speed limit signs are similar to those around school zones and are activated only at dawn and dusk. Gery says the warnings are for the protection of drivers as well as birds. Some people go out of their way to cross the bridge while the birds are swarming, he said.
“Any of them who experience the frenzy, however, likely do not drive at that time ever again,” he wrote in his email. “Because the number of birds not only makes vehicle progress difficult, but a driver who kills purple martin families probably regrets it.”
Gery says longtime residents recall purple martins congregating around the two-lane bridge since shortly after it opened in 1955. The ledges created by the I-beams offer the perfect protection from snakes, owls and other predators.
The bridge was the only route off Roanoke Island to the west until 2002, when the four-lane Virginia Dare Memorial Bridge opened, siphoning away most of the traffic. Members of the purple martin society recommend people from the island and the Outer Banks take that bridge to Manns Harbor to watch the evening martin madness from a pier at Redstone Point, at the western end of the Umstead bridge.