The mystery over who had been buying large parcels of prime real estate on Virginia's fragile and impoverished Eastern Shore has been cleared up with the declaration by a Rosslyn-based environmental group that it plans to acquire nearly 6,000 acres along the southern end of the Delmarva Peninsula.

The announcement by the nonprofit Nature Conservancy put an end to speculation that had fluorished in Eastern Shore hamlets and fishing villages since a mysterious Pittsburgh lawyer appeared on the scene seven years ago and began buying woodlands, farms and wetlands at prices well above the market rate.

The lawyer, according to a conservancy spokesman, was acting for a Pittsburgh investment partnership known as the the Allegheny Duck Club, whose principals include members of the Mellon family.

L. Gregory Low, executive vice president of the conservancy, said yesterday that the environmental group and the Duck Club recently agreed to a complex swap by which the conservancy will exchange an unspecified number of its own holdings for four duck club properties on the Eastern Shore.

Unlike other conservancy preservation holdings, which are wild and kept in their natural state, the Eastern Shore properties already contain farms and low-density housing development.

The organization said it intends to allow some additional development as long as it does not interfere with the large flocks of migratory birds present at all times of the year.

The duck club properties are two farms on the ocean side of the peninsula near the towns of Exmore and Wachapreague.

They also include acreage in and around the village of Oyster, and a parcel near the mouth of the Chesapeake Bay.

In exchange, the conservancy will give up several of the properties -- urban, industrial or otherwise not environmentally significant holdings -- that it stockpiles for use in such occasions, Low said.

Low said he believes the properties provide a novel opportunity for the conservation group, whose international holdings make up the world's largest system of private nature sanctuaries, to work in an area where development and wildlife preservation need not be mutually exclusive.

"We don't feel that this is a situation that needs locking up of large parcels of public land. So we view this as a pretty interesting and important model project -- to see if we can come up with a mixture of people and birds and fishing habitat. . . . We're not antidevelopment, but the development has to be planned very carefully and it has to be appropriate to conservation."

The Eastern Shore, situated between the Atlantic Ocean and the Chesapeake Bay, is a crucial resting point for the large flocks of migratory shore birds, songbirds and wildfowl from all over the East Coast.

"They funnel into this area," said Low. "This is the only land mass that they have at their disposal."

Low declined to say how much the acreage to be acquired is worth other than to put its value in the multimillion-dollar category.

Conservancy appraisers, he said, have only begun evaluating the properties. The swap with the duck club could take as long as three years to complete.

Low declined to say why the duck club had purchased the properties in the first place, except to say that the club's principals were "tremendous conservationists." Eastern Shore officials note that Pittsburgh lawyer William M. Robinson, who purchased the land for the duck club, had applied three years ago for residential zoning.

That request never was granted. Robinson could not be reached for comment.

The Nature Conservancy's list of major preserves in Virginia include 50,000 acres of the Great Dismal Swamp on the Virginia-North Carolina border and, on the Eastern Shore, the 60-mile-long Virginia Coast Preserve, a 35,000-acre chain of 13 barrier islands that begins south of Chincoteague Island.

Reaction to the announcement on the Eastern Shore has been positive, Low said, with officials in both Accomack and Northampton Counties in favor of the plan.

J. T. Holland, chairman of the Northhampton County Board of Supervisors, said he expects the conservancy to be a good neighbor: "Development is coming here, and I think the conservancy will help us take care of this land."

William Turner, chairman of the Accomack County Board of Supervisors, was a little less enthusiastic yesterday, saying that the mysterious land deals of the last several years had given him a jaundiced view of land deals with any outside real estate group.

"There's been a great deal of deception," Turner said, referring to long-withheld identities of the duck club principals.