THERE ARE THOSE along Virginia's Eastern Shore who snicker up her sleeves over the $5 million The Nature Conservancy spent on the 50-mile string of marshy sandy offshore barrier islands now known as the Virginia Coast Reserve.

"It's like buying a piece of the wind. "It's the way one Wachapreague waterman put it. "Those islands change all the time. They move around. Come a big storm and one of them can disappear between sundown and sunup. Once was a whole big town of people out there on Hog Island, name of Broadwater, and it's all washed away."

The Conservancy team that engineered the purchase of the 13 islands spent years untangling the skein of contradictory deeds to the 34,000 acres, more or less, on which the nonprofit organization now holds clear title. Some of the land they bought a few years ago is gone, and new acreage has appeared. The entire chain of islands is retreating toward the mainland as the world sea level gradually rises, and the whole affair has given more than one title lawyer the screaming meemies.

"This thing was a little like Br'er Rabbit and the Tar baby," a Conservancy spokesman said.'When development plans for Smith Island were announced in 1969 we decided to try to head it off because the islands are absolutely irreplaceable habitat. We couldn't let it become another Ocean City.

"But the deeper we got into it the clearer it became that the whole system is of a piece. It is a great belt of shifting sand that stretches from Delaware Bay to Chesapeake Bay, and the Virginia portion is the only relatively unspoiled section. We decided to try to get all of it that was in private ownership."

They nearly succeeded, and still are trying to acquire Assawoman Island and the remaining portions of Cedar, Hog and Smith islands, the only private land between publicly owned Wallops and Fishermans islands. The project has taken more than hours and more money (mostly from the Mary Flagler Cary Charitable Trust) than The Nature Conservancy had ever put into any single action - no small statement considering that the organization has acquired over a million acres in the past 24 years.

The Conservancy itself is a rare species among ecology pressure groups. It's oriented toward business, big business, and its top men are graduates of the world of board rooms and development. Its success in saving valuable habitats is in being able to talk the language of the owners and proprietors.

For the islands, an average of about $150 an acre was paid, much of it for sand dunes that shift like quicksilver and mosquito-infested marsh for which most people wouldn't give a dime. The Conservancy needs another $4 million, which it is trying to raise through the newly formed Friends of the Virginia Coast Reserve, to buy the remaining land, establish an island headquarters, and for staff and research.

Is it worth it? What are the islands good for, anyway?

In terms of cash the Reserve was a bargain, according to a two-year ecological-inventory study. An acre of marsh is at least three times as productive as an acre of cropland on the Eastern Shore, which in turn is the most productive farmland in Virginia.

If "raw" marsh were valued on the same basis it would be a good buy at $15,000 an acre, the study said. The reason it isn't is that those who reap the benefits - increased value of bordering lands because of the beauty of the marsh, fish, shellfish and waterfowl production, cleansing of polluted waters by natural organisms, recreation, protection of the mainland from storm tides and so on - don't realize where they come from.

Beyond that, as the report says. "The pricing of a peaceful walk along the beach at sunset, research on the geomorphology of the barrier islands, or the excitement of observing a peregrine falcon is very difficult."

It is not so very difficult for the visitor who cruises the marsh and walks Hog Island with Rod Hennessy or Barry Truitt, overseers of the Reserve. The island once was forested and long supported a town of up to 400 souls. Now two abandoned Coast Guard stations, several trash dumps and small herds of feral cattle and sheep are the only signs of man.

Seven miles long, Hog is the fastest-changing of the group. Storms in the 1930s altered the pattern of longshore sand drift and tore away much of the ground cover, and the southern end of the island began to erode rapidly. The inland town of Broadwater, then one of the Shore's major communities, was abandoned as the surf began to lap at it.

"The island was roughly a teardrop shape, with the broad end to the south," Hennessy said, drawing in the sand. "While the south end has been wearing away, the north end is building up, almost as though the whole sand bar had rotated 180 degrees."

The change is so fast that 1975 maps are already out of date. The wind whipping sand from the broad and recently deposited northern beach builds sand dunes almost while you watch; and water evaporates from the interior ponds so fast it looks like smoke.

"I think I can probably turn up a glossy ibis or two," Hennessy said, leading the way into a wax myrtle thicket. A moment later a thousand of the great birds were in the air, with a bonus of a dozen egrets, all complaining loudly.

The islands once were nesting areas and rest stops for countless millions of migratory waterfowl, which were slaughtered by makret hunters for meat and bonnet plumes; at the turn of the century Virginia canvasback ducks could be had by the barrel in New York City for a nickel apiece. Gathering the eggs of nesting gulls and terns once decimated their populations, but "egging" is declining because, as the report says, a taste for the eggs of fish-eating birds is not easily acquired.

The marsh and the island ponds remain central to the survival of the remaining waterfowl. Hog Island is littered with the corpses of Atlantic brant that starved when the past winter's extraordinary freeze covered or gouged out the shallow-water sea lettuce beds on which they depend. Others remain, months after they should have migrated, trying to gather strength to fly north.

Hennessy and Truitt seldom go out among the islands without running into trespassers or poachers, a delicate problem with no solution except, Hennessy said, "educating people to understand that we're trying to preserve the Eastern Shore way of life as well as the islands themselves."

Toward that end The Conservancy's study included a detailed survey of the people of Accomack and Northampton counties, exploring how they live, why they stay on the Shore and how they would prefer to have the islands managed. It turned out that most of them live there because they like it the way it is. Although earning a living there is hard, many said they wouldn't leave for any amount of money.

The islands have remained relatively natural for the same reason the Shore is largely undeveloped. The Virginia end of the Delmarva Peninsula is a cul-de-sac; to get anywhere from there takes a long drive to the north or an expensive one to the south via the hight-toll Chesapeake Bay Bridge-Tunnel (before that it was a long ferry ride).

Although the majority of Shore residents expressed affection for the vista the marsh and islands provide, few of them have ever visited one of the islands. Those who do go there to hunt or fish or to wrestle in private with John Barleycorn tend to feel they may do as they damn please, the same as Daddy and Grandaddy did.

The Conservancy couldn't keep such fiercely independent people off the islands if it wanted to, and for that reason has carefully chosen the name "reserve" instead of "preserve" or "wildlife sanctuary."

"Fishing from or around the islands isn't going to hurt anything," a Conservancy spokesman said. "Reasonable hunting won't either although our membership runs heavily to birders and anti-hunters in general, which has created some tension. The poaching tradition still runs strong on the Shore, and that is something we have to deal with as best we can.

"But if you arrest a local man for trespassing or poaching, you've alienated his family, friends and neighbors, and what we're hoping is that people will come to understand that the rules we have adopted are not designed to take away the benefits the islands have always given them, but to make sure the same benefits will be available to their children."

It will take years to get a handle on the processes by which the islands form and migrate, and the ebb and flow of life on them. For all that the two-year ecological study was among the most extensive and detailed that have ever been attempted on such a scale, it is largely a catalog.The plant communities, for instance, are called erra incognito in the report, and descriptions of the animal population are hedged with reservations.

"It is a researcher's paradise," Hennessy said. For instance, here on Hog Island, the wild cattle and sheep obviously are seriously interfering with plant succession. They may be the reason why the trees have never come back, and their selective grazing is obviously what permits such a profusion of thistles.

"Several doctoral candidates would have fine topics for dissertations by watching the changes that follow removal of the livestock, and somebody else could be on the beach exploring the erosion and deposition process. On other islands there are similar opportunities; in fact there is enough here to study that somebody else could work on the ecological impact of so many researchers."

He reeled in his fishing line and regarded the unmolested bait. "There's so much to be done here I feel guilty. I think I could spend the rest of my life watching the seasons and the wildlife come and go."