Those people you see peering intently at old brick chimneys at dusk this time of year are awaiting one of nature’s great spectacles, the yearly arrival of chimney swifts.

Flying in flocks of a few birds to more than 1,000, the 4- to 5-inch swifts (Chaetura pelagica) select a chimney, swarm in great swooping circles around it, then drop precipitously into it for a night’s rest.

In the daytime, they are one of nature’s prime vacuum cleaners, scarfing up all kinds of annoying insects on the wing. At dusk, their close-quarters acrobatics generate awe in bird-watchers and intense interest in scientists. Some at UNC-Chapel Hill are studying them on behalf of the Navy.

In Charlotte and Raleigh, they’re stopping over as they move from chick-raising season along the East Coast to wintering in South America. Most will likely be gone after next week.

Around Charlotte, some of the places they’re being seen are Dilworth Elementary School, Shamrock Gardens Elementary School, Lake Norman Christian School, the Carolina Inn at Davidson College, North Mecklenburg High School and Sun Valley High School in Indian Trail.

“Sundays in September with Swifts,” sponsored by Wake Audubon and the N.C. Museum of Natural Sciences, has become such a tradition in Raleigh that bird lovers bring their lawn chairs and bottles of wine to socialize as they watch.

Some flocks are being seen at Edenton Street United Methodist Church, the Professional Building, Stone’s Warehouse in east Raleigh and Fuquay-Varina Middle School.

Winging to oblivion?

The swift, named Audubon North Carolina’s focus bird for 2016, is declining steadily in numbers. The Cornell Lab of Ornithology says the cumulative drop since 1966 is 65 percent.

One suspected culprit is the disappearance or remodeling of old brick chimneys where the birds nest and, during migration, roost.

Swifts are unable to perch like other birds, and must cling to a vertical surface to build a nest or even to get some rest.

“There’s data to suggest they may sleep some on the wing,” says John Gerwin of the N.C. Museum of Natural Sciences in Raleigh.

To build a nest, they use their momentum to snap twigs in flight, then secure them to chimney interiors with saliva.

Metal chimney liners leave them nothing to cling to; chimney capping shuts them out; and heat pumps make chimneys unnecessary.

In Wake County earlier this decade, swifts roosted in 32 school chimneys, Gerwin said. “The last time we looked it was nine.”

Some solutions

Wake Audubon and the N.C. Museum of Natural Sciences in 2014 built a 30-foot, $36,000 roosting chimney at Prairie Ridge Ecostation in West Raleigh.

Smaller wooden or masonry towers for nesting, 8- to 12-feet high, are being built in backyards and parks by individuals and Boy Scouts. N.C. Wildlife Federation’s HAWK (Habitat and Wildlife Keepers) is offering $300 construction grants.

When buildings that house large roosting chimneys are renovated, owners are being asked to save chimneys for the birds’ use – or offer alternatives.

John Connors, former Wake Audubon president, lists five pending renovations at Raleigh roosting sites, including the Dillon Supply Co. warehouse downtown and Stone’s warehouse in east Raleigh.

As a preliminary to promoting chimney preservation, Mecklenburg Audubon president Ken Kneidel is asking members and the general public to document when and where they see swifts roosting, through a form on www.meckbirds.org and at Cornell University’s bird website: www.ebird.org.

One fan of the birds is Raleigh developer Greg Hatem.

For years, his children watched eagerly from their downtown home as thousands descended on his Odd Fellows Building nearby.

“It was like an old friend was coming up to see them. They’d see the first one and start running through the house – ‘We’ve got to go see them!’ ”

“It’s pretty great to see nature like that, especially in the downtown,” he said.