Restoring a species to its native habitat is usually considered a good thing, but an unusual study reconstructing historic bald eagle diets is raising flags over their reintroduction off the California coast.

Attempts to bring back the United States' iconic bird to the Channel Islands could put at risk populations of recovering seabirds and the threatened island fox, according to research published in Monday's Proceedings of the National Academy of Sciences.

A bald eagle nesting on California's Channel Islands in 2008. (Peter Sharpe/Institute for Wildlife Studies)"When you reintroduce an animal to a place it used to occur, it's important to understand what sort of interaction they had in the past," said lead author and animal ecologist Seth Newsome, a post-doctoral researcher at the Carnegie Institution of Washington at the time of the study.

"A lot of the time we think they're perfectly suitable, but it turns out the ecosystem has changed dramatically since they were last there."

Newsome and his colleagues analyzed carbon and nitrogen isotopes of fossilized bones from bald eagle nests on the islands dating as far back as 30,000 years. Their analysis shows that eagles fed mainly on seabirds until the last breeding pair disappeared from the islands in the mid-1900s.

Bald eagles were nearly wiped off the face of the continental U.S. and parts of southern Canada, largely due to the pesticide DDT which thins eggshells and kills chicks before they hatch.

But protective regulations in the U.S and Canada, beginning in the 1970s, led to significant reductions in DDT and marked the beginning of the eagles' comeback.

Now that the eagles are being reintroduced to the Channel Islands, they could "exert significant predation pressure on the fragile fox and seabird populations," says Newsome.

The problem is that while once predator and prey were in balance, that's no longer the case.

When European settlers arrived on the islands in the mid-1800s, they brought with them sheep, which overgrazed the land. That lead to vegetation loss, large-scale soil erosion and unstable sand dunes, all of which ruined the seabirds' ground nesting sites and caused their numbers to plummet.

As the seabird population declined, the eagles turned to sheep. While they didn't usually kill the sheep outright, they fed off their carcasses, especially during lambing season when mortality rates were highest, says Newsome.

But starting in the 1960s, the U.S. National Parks Service removed non-native animals, including sheep, from the islands in an attempt to restore the area to its natural state. Seabirds, including the Cassin's auklet and the eared grebe, slowly began returning to the islands.

"But the seabird populations haven't recovered to the level needed to sustain a growing and healthy bald eagle population," says Newsome.

Starting in 2002, federal authorities began reintroducing bald eagles to the Channel Islands and in the past four years some birds have hatched chicks.

While in isolation, that is cause for celebration, Newsome cautions that it may have significant consequences for other creatures that call the islands home. He worries the eagles may supplement a limited menu of seabirds with the threatened island fox, threatening both creatures.

"Right now, there is not a very systemic effort to figure out what these eagle are eating," said Newsome. "We need to be concerned about their impact on other species in the ecosystem."