When evening's last light slants low over the Voorhies mansion roof, the little black birds flitting about EdenVale Winery begin to assemble for their nightly spectacle.

The mass of Vaux's swifts that had been zipping around the winery slurping mosquitoes pack themselves into black sheets that dart in unison through the sky, slowly inching toward the mansion's old brick chimney.

"They're going to swirl around and tease us," says biologist Karen Hussey, clicking counter in hand, ready to quantify the evening. "It seems like nobody ever wants to be the first."

Then one swift darts into the sun-bathed chimney and soon the whole flock swirls into it en masse like a genie going back into the bottle, with Hussey feverishly clicking away.

Tomorrow they'll be gone, replaced by another collection of swifts that have learned through generations that the Voorhies chimney is their Southern Oregon host during spring and fall migrations.

The mansion's chimney is Jackson County's lone known roost site for migrating swifts, and Hussey is launching a volunteer effort to chronicle when and how many of these hummingbird-like birds use it.

It's a local version of a citizen-science effort that is tracking Vaux's swifts up and down the West Coast through the online "Vaux's Happening" effort organized by Larry Schwitters, a retired middle-school science teacher who has been tracking swifts since 2008 (vauxhappening.org).

Volunteers like Hussey help track the relative abundance of Vaux's swifts, chronicle the importance of chimney roosting sites and, over time, help create an index to tell whether the population now estimated at 340,000 birds is increasing or dropping.

Pronounced "vawks," Vaux's swifts are voracious insect eaters that fly all day without perching because their feet cannot grab onto a branch or wire. Instead, the 4-inch birds are designed to hang off vertical surfaces.

During migrations, they'll fly 100 miles or more before roosting at night. Built sleek for acrobatic flying, their bodies can't fluff up at night to keep warm, so they cling in tight colonies inside structures like chimneys to keep warm, Schwitters says.

They historically used hollows in large trees such as Douglas firs and redwoods, but they have adapted over time to value man-made structures like chimneys, almost exclusively those built with brick and mortar before slick chimney liners became the norm in 1941, he says.

"What we've discovered is the reason the birds go into chimneys is largely to keep warm," Schwitters says.

Chimneys soak up radiant heat during the day and emit it at night, aiding swifts in their warming efforts, he says.

The birds breed in summer, and the fall migration south to Central America contains young birds learning where the preferred roost sites are from the adults, Schwitters says. They return to the roost sights in spring as they retrace their nearly 4,000-mile journey, he says. 

Schwitters says he and other volunteers have discovered that a dozen chimneys across western North America house nearly 85 percent of the migrating swifts. Sites like the Chapman School chimney in Portland will get more than 4,000 birds some nights.

There's not a lot of data on the Voorhies site, but so far it ranks 87th out of 133 known Vaux's swift roost sites, he says.

Mansion and winery co-owner Anne Root, who bought the property in 1999, discovered the mansion also has a remnant year-round population.

"We even have little ones hatch and fly around in the mansion," she says.

The lighter spring migration through Voorhies runs April and into May, Root says. The heavier fall migration is a regular floor show for evening wine tasters from August into September, she says.

"That's when everybody turns their chairs around and watch the swifts go down the chimney," Root says. "It's beautiful."

To help the efforts of Schwitters and other swift lovers, there needs to be more concrete data. That's where Hussey comes in.

She wants to assemble enough volunteers to chart use of the 19th-century mansion's chimney during the spring migration and again during the fall session now winding down.

Hussey shows up at the winery about 20 minutes before dusk. She logs the date and temperature and notes whether any predators such as hawks or peregrine falcons are hanging around in hopes of a snack.

The birds are scattered throughout the area but accumulate at dusk. They invariably start darting down the chimney within five minutes of sunset.

This particular evening sees the first swift followed quickly by 106 more. After a short respite, the final swift drops into the chimney for a night at the mansion.

"It's a little different every night," says Hussey, a land steward with the Southern Oregon Land Conservancy. "But it's always incredible."

Reach Mail Tribune reporter Mark Freeman at 541-776-4470 or mfreeman@mailtribune.com. Follow him on Twitter at www.twitter.com/MTwriterFreeman.