SOUTH KINGSTOWN, R.I. (AP) _ The only frolicking the federal government wants on a nude beach in Rhode Island is by a small group of birds whose existence is considered threatened.

The U.S. Fish and Wildlife Service has decided that the secluded and popular Moonstone Beach will be off-limits for nude sunbathers this summer in order to protect the tiny piping plover.

Moonstone has been the only spot on the 420-mile Rhode Island coast - and one of few along the New England shore - where nudists can sun in the buff.

But federal officials and environmental groups say that when humans enter, the piping plover exits. The bird is listed on the federal endangered species list as threatened, a step away from the designation of endangered.

''The human activity serves as a distraction to the plovers,'' said James Kurth, manager of the Trustom Pond National Wildlife Refuge, which includes Moonstone Beach. ''They're constantly moved away from their breeding grounds.'' He cited a University of Massachusetts study more than a decade ago that showed plovers will leave an area where humans congregate.

Nudists see the battle over the bird as a subterfuge to the underlying issue: They claim the government wants Northeastern shores free of nude sunbathers.

They also say they can coexist with the little bird - and, in fact, have for years.

''It doesn't have anything to do with the animal unless the people are bothering them,'' said Ron LaPorte of Glastonbury, Conn., as he lounged on a blanket on the sun-drenched beach Thursday morning. He was bundled in a denim jacket against a sea breeze.

''Birds are important, but the public is important, too,'' said Dave Gillis of Warrensville, Conn., also sunning himself on the beach. ''We paid for this beach. ... They've got to consider the people.''

The controversy over Moonstone has been simmering for years. But when the Fish and Wildlife Service expanded its holdings in 1982, nudists and clothed beachgoers began to feel a little more unwelcome.

For one thing, their beach kept shrinking.

In 1982, the federal government obtained land from the Audubon Society and took control of about a mile of the coast, Kurth said. The following summer, about three-quarters of a mile was closed. A fence erected last year left a thin strip along the water's edge for sunbathers.

This year, when the plovers come back around the last week of March, that fence will go to the high tide mark along the entire length of beach to keep sunbathers - clothed and naked - out.

''We're applauding them very loudly,'' said Alfred Hawkes, executive director of the Audubon Society. Hawkes said his group had objected to the nude sunbathers, but had been unable to keep them out.

Hawkes said the sandy strip of coast appears to be ideally suited to plovers, so tiny that adults reach only 7 1/4 inches. On one side lies the blue expanse of ocean. Beyond sand dunes on the other side is the freshwater Trustom Pond.

In the early 1970s, piping plovers and least terns were nesting there regularly, Hawkes said. ''It got pretty much out of hand in the late '70s. Once the word got around among nudists the crowd grew very quickly.''

Nudists said they are being branded unfairly as anti-bird.

''There seems to be some bias functioning here, although it's denied by officials,'' said Lee Baxandall of Osh Kosh, Wis., president of the nationwide Naturist Society. ''There is no federal law against nudity.''