A nest of streams, rivers, fields and forests, the Prince William area is prime territory for watching birds.

It's "just a good area with all its different habitats," which draw a wide range of species, said Nokesville resident and ornithologist Ken Bass, who has seen 239 species of birds in the Prince William area, including a golden eagle, Brewer's blackbirds from west of the Mississippi River, the rarely seen Henslow's sparrow and the Lapland longspur, which breeds in the Arctic tundra.

The only jurisdiction on the Atlantic seaboard that stretches from sea level to mountain crest, Prince William offers diverse habitats for birds, particularly in its rural western end. The county is included in Virginia's Piedmont area, which extends roughly from the Blue Ridge Mountains to Interstate 95.

"It's still quite unspoiled," said Lois Burke, of the Virginia Society of Ornithologists. "With its vast and open fields and woods, it {offers} a marvelous opportunity to see species" that shy away from Prince William's more developed northern neighbors.

Reaping the harvest of Prince William's ideal location are "birders," as birdwatchers are called, who stroll area fields and forests, scour odd spots, such as the Independent Hill landfill or Manassas Airport's tarmac, and occasionally clog rural roads in their quest.

"The great thing out here is there are so many {back roads} you can drive along," said Manassas resident Tanis Moher. She went on Ken Bass's field trip to the rural Nokesville area Saturday with a small group from the Fairfax chapter of the National Audubon Society, which has about 160 members from Prince William.

The tour made its way from Brentsville, up Route 619, down Valley View Drive onto Parkgate and Aden roads and into the Quantico Marine Reservation, where wild turkeys can sometimes be seen barreling across the roads. Among last Saturday's sightings in the frigid, overcast skies were six species of hawks, including numerous kestrels, a black vulture and an immature red-shouldered hawk. Hooded merganser ducks also were observed on several ponds.

The caravan of four cars on Bass's tour pulled on and off the back roads, stopping long enough for the watchers to get eyefuls of a bird, sometimes perched on a power line or a farm fence.

"All the farmers are driving by, saying, '{Blank} bird-watchers,' " said Charles Chambers, the Fairfax Audubon Society's chairman of excursions, as one pickup truck zoomed around the cars.

Birders on a field trip led by Bass for the Virginia Society of Ornithologists in mid-October saw 61 different species, including the state's endangered loggerhead shrike.

"That was just a half-day trip too," Bass said. Bass has helped place Prince William on bird-watchers' maps with numerous birding records, including the Piedmont area's first official viewing of breeding blue-winged teals.

"You couldn't dream of doing this in Arlington. You'd have to go to a park," Moher said Saturday, preparing to duck into a dark silo where several barn owls were snoozing. The tour stopped at several farms along the route.

"The barn owl -- that's one bird that can't be missed on a Nokesville trip," Bass said. Bass is fighting to preserve the county's declining population of barn owls by placing handmade nesting boxes at the tops of silos.

Most Prince William area birders are likely to be more casual with their hobby than Bass, according to local birders. Bass and his wife, Fran, have converted their Nokesville home, a stop on Saturday's field trip, into a bird haven with numerous bird houses, several piles of brush and a tiny pond.

In the United States, there are about 61 million birders, which exceeds national numbers for hunters and fishers, according to an article in this summer's edition of "American Birds." But only 3 percent of the birders are "committed," which means they spend an average of $1,852 annually on birding travel, equipment and supplies, according to the article.

Birding "is just a nice, pleasant thing to do," said Martha Smith, of Lake Jackson, whose husband, Ralph, went on Bass's tour last weekend. "We're not gung ho . . . to see a whole lot of species. We don't keep a list and all."

The Smiths do, however, take their "cheap" binoculars with them everywhere they go. "You can see something any time," she said, explaining that their birding locations include Possum Point, where they have seen bald eagles and tundra swans. Canoeing up Lake Jackson, the Smiths frequently see osprey, prothonotary warblers and wood ducks.

The best birding strategy, she said, is "to pick a good spot and go back there and see how things change."

At the Independent Hill landfill, diligent birders can see seven different species of gulls, including the lesser black-backed, the glaucous and Iceland gulls, according to Bass. "People who go to the landfill . . . think, 'Oh, a bunch of seagulls.' . . . But there are seven . . . whole species," said Bass, who has also seen 22 shore species in the county.

Another good birding strip is the Manassas Airport. At dusk, scavengers, such as the short-eared owls, northern harrier and rough-legged hawk, skim over the runways in search of prey, according to Bass.

Trails through Bull Run, Prince William Forest and Manassas National Battlefield parks as well as other parks and local waters are good places for birding. A bald eagle nest on Powells Creek's northern shore is visible from Leesylvania State Park. And just across the Occoquan Bay in Mason Neck State Park and National Wildlife Refuge, there is a 772-nest blue heron rookery.

Free birding walks in the Prince William-Fairfax area are sponsored twice a month by the Wild Bird Center in Woodbridge, a specialty shop for bird supplies and birding equipment. Their field trips are led by local birder and nature photographer Jack Nevitt. For information on the field trips, call 490-5000.

Group field trips are especially beneficial to people new to an area or to birding novices, according to Bass, who will lead field trips on request.

Judy Kenyon, of Dale City, who has been on several outings with Bass, agrees. "I'm learning all the birds who don't come to my feeder," she said.

The Fairfax Audubon Society's field trips are open to the public. For information on future trips, call 642-0862. Birders interested in participating in the National Audubon Society's annual Christmas count of migrating winter birds, which begins Dec. 16, should also call that number.

The Virginia Society of Ornithologists, which costs $3 to join, has some field trips open to the public. Call Lois Burke at 524-9428 for information.

For information on rare area sightings, call the Virginia Bird Hotline at 804-929-1736 or the Audubon Naturalist Society of Central Atlantic States at 301-652-1088.