You've flown 100 miles since sunup, pushed by fall's first cold snap from the northwest. Your silent companions survey the landscape below, a bright patchwork of farmland and forest. You wheel slightly west, toward a sandstone outcrop, and spy something moving in the afternoon light. By your instincts as a hunter, you drop closer.

Whoa. The creatures moving below, holding strange black eyes against their faces, are 10 times your size. Belatedly, some part of your brain recalls seeing them on your flight south. You pull up, flap your wings once, and soar gracefully south, past the annual flock of bird-watchers clustered atop Hawk Mountain.

Each fall, 18,000 raptors get a bird's-eye view of this central Pennsylvania sanctuary, located an hour northeast of Harrisburg, Pa. They're on routes that can begin at Canada's Hudson Bay and end in Argentina, soaring down the Appalachian Kittatinny Ridge before cutting across Texas into Mexico, where they mass in the millions.

Birders do some massing of their own this time of year, and Hawk Mountain Sanctuary is one of the best places in the world for humans to watch hawks fly south. Every September and October, researchers and amateurs at this nonprofit refuge track the flight of 16 species, from tiny kestrels to bald eagles, as part of the longest ongoing record of raptor populations in the world.

Migration is hard work. But hawks, falcons and eagles make it look easy, gliding at eye level past the hilltop sanctuary in numbers that can reach 1,000 per day. And it's easy to copy their laid-back style by kicking back for an afternoon on the sunbaked boulders.

We've come to Hawk Mountain not as ornithologists but as gawkers, savoring the late summer views and the fellowship of bird lovers. The north-south ridges of central Pennsylvania and the valleys in their folds look like a rumpled Indian rug, streaked with vivid orange and red as the birds streak overhead.

Thousands of folks visit the sanctuary a year, but it's not the biggest local draw. That would be Cabela's, in nearby Hamburg -- the outdoor gear superstore that draws millions of visitors each year. But the store's location is a clue to the rugged lure of this terrain along the Schuylkill and Little Schuylkill rivers.

From the sanctuary's 1,300-foot altitude, the view can extend some 70 miles. The exhilaration of the setting is the hawk's perspective: With nothing in front of you and the valley, you can easily imagine gliding right off the side of the mountain. That's a genuine hazard for the hikers along the notoriously difficult Pennsylvania section of the Appalachian Trail.

"We get a lot of Appalachian Trail hikers -- it runs right through town," says Jack Boran, owner of the Port Clinton Hotel, a watering hole near Hamburg. Like a discerning bird-watcher, Boran can distinguish the plumage of visitors. "The birders might wear a shirt from Hawk Mountain or the Audubon Society. The hikers are wearing packs and boots -- and you can smell 'em," he jokes.

On the Saturday we visit, an early-morning shower shrouds the vista, and as a result we meet our first hawk not over the valley but in the sanctuary's visitor center. A steep two-mile drive from a hex-painted barn on Route 895 brings us to the low-slung building, set across the road from the lookouts and trails.

Reaching into a cage for a chirping broad-wing hawk, educational specialist Jeremy Scheivert gives us the raptor rap sheet; "Raptors vary in size but have two common physical features. First, talons, sharp toenails, good for catching and killing their food. Second, they can see very clearly at long distances."

Our eyes being somewhat less acute than raptors', we seize the Hawk Mountain field guide that classifies hawks by shape. Buteos sport a classic broad-winged, round-tailed silhouette. Falcons have pointed wings and long, tapering tails; accipiters, small wings and long tails.

The rain ebbs as we stroll the flat path to the nearest observation point, South Lookout. A bulletin board with handwritten highlights tells us we should have been here yesterday: six bald eagles were sighted, a season record. The view opens from the trees like a children's pop-up book onto a deeply peaceful valley: rivers, farms and forest. The first hawks of the day, in groups known as kettles, spiral over the ridgeline as the air warms.

"When there's a cold front over the Appalachians, we usually get northwest winds, and birds conserve energy by riding that deflected air current," says Scheivert. "Their other strategy is riding thermal convections. With the sun beating down on the valley, a hot column of air rises up. If you're a broad-wing hawk, you get in that column of air and circle, and the heat actually lifts you. When the hawks are high enough, the heat dissipates. No more lift. They turn their nose, point south, and they sail."

The white streak through the center of the valley is the River of Rocks, boulder-size leftovers from the glacier that pushed past 11,000 years ago. We could have hiked a four-mile circuit around the formation. Instead we opt to hike only another 200 feet to the North Lookout. The slight exertion brings us to the Slides, where conservation history and hawks converge.

This stone ledge provided good shooting for the local gentry; a vintage photo in the visitor center shows hundreds of dead birds covering the ground near the North Lookout. In 1934, a New York conservationist named Rosalie Barrow Edge raised funds to buy 1,400 acres for a sanctuary.

Eagles, the celebrities of the raptor world, are now a conservation success story. Their recent counts, Scheivert says, have consistently surpassed previous records. But not all hawks are rising; smaller birds, including kestrels, seem to be in decline.

As the rocks heat up, smaller creatures hover -- monarch butterflies, dragonflies, hummingbirds. We could fill the rest of the afternoon with other pursuits: Renninger's, the Kutztown antiques market, is a half-hour away. But like most visitors, we prefer to stay on the mountain as long as the light lasts, then head back toward Route 61 for dinner.

The Yuengling beer at Michael B's in Orwigsburg is frosty and fitting, not only because it's brewed up the road in Pottsville. The bald eagle on the label reminds us that he's still missing from our life list -- a reason to return, like the raptors, to Hawk Mountain each fall.

Each year, enthusiasts flock to Hawk Mountain to view the annual raptor migration. Above, a mounted owl decoy attracts passing hawks.