The Nature Conservancy announced yesterday that it is buying and protecting 1,313 acres of ecologically valuable land on North Carolina's Outer-Banks and obtaining conservation easements to another 5,000 acres.

The land, located on Swan and Monkey Islands 20 miles south of the Virginia-North Carolina border, is a favorite stopping point for transient birds on the Atlantic flyway, a sort of interstate air route for migratory waterfowl.

The Conservancy said it would buy the property, which it described as "two of the finest undisturbed areas" on the Outer Banks, with a $4 million grant from the Pittsburgh-based Richard King Mellon Foundation. The Conservancy said the donation is the largest ever made to a conservation organization.

The purchase of the easements protects the land, which is comprised mostly of marshes and shoal land, from any development, according to a Conservancy spokesman. The land is in an area that is "under a great deal of pressure from developers," he said.

Actually, years of tidal movement and shifting sands have left the Swan and Monkey tracts islands in name and memory only. Now they are part of the long narrow stretch of marshlands and sand that stretch south from Virginia Beach to Cape Hatteras and from ther, after a brief interruption of water, to Ocracoke, N.C.

The islands sit sedately between the Atlantic Ocean and Currituck Sound, a location that makes them intensely popular with about 170,000 birds that make it an annual winter stop on their migratory routes along the Atlantic flyway.

More than 100 species of birds have been spotted in the area, according to Conservancy officials, including the peregrine falcon, osprey, southern bald eagles, ringnecked ducks, widgeons and canvasbacks.

The Conservancy is purchasing Swan Island from the Swan Island Hunting Club, a group that was formed in 1870 when a small band of hunters was stranded aboard the "Anonyana" in upper Currituck Sound. Unable to return to the warmth and safety from whence they sailed, the hunters turned their attention to the surroundings birds instead, with such happy hunting results that they decided to make a habit of returning.

The members of the club will continue to be able to hunt there, a Conservancy spokesman said.

A private, nonprofit group that acquires and preserves ecologically valuable land, the Arlington-based Conservancy has aided in the preservation of more than 1 million acres of land nationwide in the last 25 years, including land in the Great Dismal Swamp, inlands in the Potomac River, a forest in Georgia, and one of the favorite haunts of the nesting sea turtle in Florida.