Digital Extra

In the months after the barge North Cape gushed some 800,000 gallons of diesel heating oil on one of the rare birds' last breeding grounds, the poor breeding results have raised a troubling question: Has the winter oil spill harmed the ecosystem more than it first seemed?

Scientists, who have taken egg samples from the failed nests and are in the midst of testing the remains, stress that it's too early to tell if the plummeting breeding performance is related to the spill.

Nonetheless, bird lovers and biologists are on the trail of a peculiar wildlife tale.

"We need embryologists to examine what might be in the eggs for deformities that were perhaps caused by petroleum hydrocarbons," said Charles Hebert, the manager of Trustom Pond National Wildlife Refuge. "We don't know yet what we will find, and we may never find out."

The piping plover is considered a threatened species -- just a step away from endangered. But scientists say that since 1986, when the plovers became a protected species under the federal Endangered Species Act, the birds in New England have begun a "spectacular recovery."

In Rhode Island, the number of pairs nesting doubled from 19 pairs to 40 pairs from 1989 to 1995.

The success had been particularly marked at the South County wildlife refuge, a near-pristine habitat of clean dunes and tranquil salt marshes. In past years, the refuge's breeding population has produced on the order of 18 chicks each year.

This year it appears the number is nine, due to low egg counts and nest abandonments by four pairs -- an alarming event for the usually defensive plover parents.

The birds also exhibited unusual choices for nest selection, including a nest at Green Hill Beach and a nest on Block Island, where no nesting plovers had been recorded since 1973. Hebert said these pairs may have been plovers that abandoned the traditional Moonstone site, but again he said he's not sure.

The eggs in the Block Island nest never hatched, even though they were closely monitored, according to Christopher Littlefield of the island's Nature Conservancy.

"It had been a cold spring," he said. "We didn't want to believe it. Maybe the textbooks are wrong."

Scientists are also considering other possible factors:

--A dearth of proper nutrition for the parents. This, too, might be related to the spill, which killed much of the local food chain, at least temporarily.

--Storm surges, such as Tropical Storm Bertha, whose waves claimed one chick.

--Human interference, such as vandalism to nesting areas, or the presence of unleashed dogs near the nests.

--A very cold spring.

--Minks, coyotes and rats.

--Predators that don't leave tracks, especially birds that could harass the enclosed nesting areas until the parents abandon it.

Some factors might also be working in concert, conspiring to keep the chick numbers low. Indeed, eggs produced by one pair were smashed by a dog. Four others were eaten by a coyote and four by a mink, officials say.