Each winter for years, hundreds of migrating trumpeter swans have arrived in southeastern Idaho, finding everything they need to carry the pure white birds over until spring.

The birds traditionally arrive in the Henry's Fork area of the Snake River Basin in early December, attracted by the remote and rural area's ice-free waters just west of Yellowstone National Park, the lush aquatic plants and few humans to bother them.But the area apparently became too popular, drawing in about 700 birds of the entire 2,000 wintering population of trumpeter swans from Canada and the western United States to Idaho's Harriman State Park on the Henry's Fork River.

More than 100 of the rare swans died in February 1989 when Henry's Fork froze, U.S. Fish and Wildlife Service officials said. Emergency releases of water from Idaho's Island Park Dam melted the river and gave hundreds of weakened swans access to needed forage.

Then, last year, too many swans in the Harriman State Park area exhausted the food supply by early March, the service said. The aquatic plants have not recovered, and the agency believes there will not be enough forage to sustain the expected 1990-91 winter swan population until next spring.

Service biologists, joined by wildlife crews from Idaho, Utah and Wyoming, and Bureau of Reclamation and National Park Service workers, will attempt to trap as many trumpeter swans as possible in early December and relocate them to more suitable wintering sites in Utah, Wyoming and southern Idaho.

About two dozen trumpeter swans will be released at Fish Springs, a federal wildlife wetland refuge surrounded by desert in western Utah, said Steve Phillips of the Utah Wildlife Resources Division.

No swan or goose hunting occurs at Fish Springs, Phillips said. But the trumpeter swans still will be marked with dye to make them more easily distinguishable from tundra swans, which are hunted on a limited basis in Utah.

The crews will attempt to scare off swans that cannot be captured, as part of the effort to reduce wintering populations at Harriman State Park. Biologists hope hundreds of the swans can be trapped or hazed southward to new wintering areas with better food supplies and higher chances of survival for the rare birds.