The 52-mile long mountain, among the longest in the Appalachians, helps produce thermal air currents that give migrating birds the lift needed to glide across vast segments of land while expending small amounts of energy.

Mid-September is generally the peak migration period for the broadwing hawk, the most frequently seen southbound raptor riding the thermals over Peters Mountain. On sunny September days when steady winds are blowing from the north, the numbers of birds of prey spotted here can reach into the hundreds -- even the thousands. The single-day record for broadwing sightings here was 2,684, set in 1974.

The group of overhanging sandstone summit slabs known as Hanging Rock gives hawk watchers sweeping views of the adjacent Sweet Springs and Potts Creek valleys and points beyond.

The Peaks of Otter along the Blue Ridge Parkway 50 miles to the northeast can be seen from here, along with Cold Knob and the Beech Ridge wind farm near the Greenbrier-Nicholas County border, a near equal distance to the northwest.

Among the first hawk watchers to scan the skies from this 3,812-foot-high Monroe County vista were Charleston area members of the Handlan Chapter of the Brooks Bird Club and biology students led by Dr. Ralph Edeburn of Marshall University and Dr. Paul Cecil Bibbee of Concord College. Since 1952, hawk watchers at Hanging Rock have also kept track of the numbers and types of hawks, eagles, ospreys and falcons seen migrating through.

"Back then, the count year may have involved only one or two days of observations," said Rodney Davis, who was scanning the skies for migrant raptors at the site on Tuesday. These days, volunteers tally southbound birds of prey from mid-August to early December. Detailed records, including breakdowns of the species counted and weather data, are logged daily and posted by Davis on the website (www.hangingrocktower.org) of the volunteer group.

In the early 1950s, volunteer hawk watchers spent their time at Hanging Rock exposed to the weather, perched atop the lichen-draped sandstone summit boulders. That changed after 1956, when the state Division of Forestry established a fire tower on the site, located in an 18,530-acre segment of the Jefferson National Forest that spills into Monroe County from Virginia.

The state abandoned the fire tower in 1972, and its upkeep was taken over by the Handlan Chapter of Brooks Bird Club. The tower cabin got a new roof and a major facelift in 1984, thanks to a grant from the state's Nongame Wildlife Program. But in 1996, the year after a new deck was added, vandals burned the building. The current window-lined tower cabin, now known as the Hanging Rock Raptor Observatory, was built the following year, and completed in time for monitoring the 1997 fall migration.

The Jefferson National Forest added new stairs and new interior paint to the observatory earlier this year. A new composting toilet is expected to be added soon, to replace a homespun outhouse that was cited for its outstanding view in the guidebook "Way Out in West Virginia."

Jim Phillips, the naturalist at Pipestem State Park and a frequent hawk watcher at Hanging Rock, sends migration data from the Monroe County site to the Hawk Migration Association of North America. The Hanging Rock Observatory is one of nearly 200 hawk-watching sites in the U.S., Canada and Mexico that contributes migration data for the annual Raptor Population Index, produced by the HMANA and three other birding groups with support from the U.S. Fish and Wildlife Service. Hanging Rock is the only site in West Virginia that contributes data to the HMNA.

With decades of raptor migration data in Hanging Rock's logbooks, some population trends have become evident.

"We're seeing a lot more eagles than we used to," Davis said. In the 1970s and early 1980s, it was not uncommon for observers at Hanging Rock to fail to spot a single bald eagle or golden eagle during the course of a year. In 2010, a record 137 bald eagles and 54 golden eagles were seen.

A number of non-feathered flyers can also be seen at Hanging Rock Tower. Military aircraft frequently make low-level training runs over the area, and glider pilots use the thermal currents rising off Peters Mountain to make long-distance flights.

Eighteen volunteers are deemed to be sufficiently schooled in raptor identity to qualify as counters and add data to the daily logbook.

"We're not really an organization," Phillips said. "Everyone's a volunteer, and people go to the tower whenever they are able. We try to have people up there as often as possible, especially during the peak of the broadwing migration in mid-September."

Visitors are welcome any time at Hanging Rock. The tower building is public property, maintained by the U.S. Forest Service. During the fall migration, visitors watch hawks on their own, or help spot incoming birds for those trained to identify them.

"We average about 1,500 visitors a year when we're up here," Davis said. "I enjoy meeting new people at least as much as I like seeing hawks."

The rocky, mile-long walk to the tower along the Allegheny Trail from a parking lot off Limestone Hill Road didn't keep 12 visitors from appearing at Hanging Rock on Tuesday. Among those spending time at the tower were Franz Behmer and Annelie Bauman of Bayreuth, Germany, and their local hosts, Kevin and Holly Harvey of Greenville, and Oak Hill area residents B.J. Bernath and Steve Piotrowski, who motorcycled to the Hanging Rock trailhead.

"I love it up here," said Bernath, who was making his third trip to Hanging Rock, shortly before a bald eagle glided into view, corkscrewed its way upward in a thermal, and soared southward after gaining several hundred feet of elevation.

"I've seen bald eagles when I was living in Colorado," Piotrowski said. "But this is my first West Virginia eagle."

Having foreign visitors at Hanging Rock is not all that rare, according to Davis. "We had an Austrian boy up here earlier this season," he said.

After posting a season-high daily count of 187 raptors on Monday, Tuesday's tally dropped to eight, because of prevailing winds shifting from the northwest to the southeast.

"The birds don't want to expend any more energy than they have to, so they'll wait until they won't have to be flying into the wind before they move on in bigger numbers," Davis said.

Many of the hawks, including most of the broadwings, travel to South America via Texas to overwinter.

"Broadwings are the only raptor that migrates in large numbers," Davis said. "It's usually around Sept. 20 that the largest numbers of them are flying over here."

When mid-September flying conditions are optimal, hundreds of broadwings can be seen gliding overhead in swarms, called kettles, as they ride thermal updrafts over the crest of Peters Mountain.

"It's like a swarm of bees, only it's hawks," Davis said. "There can be two or three hundred or more hawks in a kettle. That's what people come up here to see."

Last year, observers at Hanging Rock tallied a total of 3,966 raptors. The most counted and identified in a single year was 10,572 raptors tallied in 1974, during the tower's record-setting broadwing migration.

But people can make the trip to Hanging Rock and not see any migrating birds, if atmospheric conditions are unfavorable.

"I tell people to come for the view," Davis said. "If they see a lot of birds, too, that's a bonus."

One way to reach the Hanging Rock Tower from Charleston is to drive to Lewisburg via Interstate 64, and follow U.S. 219 south to Union. At Union, turn left (south) on W.Va. 3 to the outskirts of Gap Mills and watch for a brown "Watchable Wildlife" sign on the right, at the turnoff for Zenith Road. Follow Zenith Road about 3.5 miles to its junction with Limestone Hill Road. Turn left on Limestone Hill Road and follow it about 1.8 miles to the top of Peters Mountain and a Forest Service parking area for the Allegheny Trail. Hike the Allegheny Trail south about one mile to Hanging Rock Tower.

For more complete driving directions, maps, photos and annual migration totals, visit www.hangingrocktower.org.

Reach Rick Steelhammer at rsteelham...@wvgazette.com or 304-348-5169.