A growing population of American bald eagles in Channel Islands National Park might eventually start feasting on rare seabirds and endangered island foxes, researchers reported Monday.
The warning was based on an extensive analysis of the shifting diets of the opportunistic foragers from the Pleistocene era, about 20,000 years ago, to the late 1960s, when they were decimated by widespread use of DDT. It was reported in the online Proceedings of the National Academy of Sciences.
An ongoing National Park Service restoration effort has successfully reestablished an estimated six or seven pairs of bald eagles on the islands off the Ventura County coast, federal wildlife authorities said. The population could reach a historic high of about 25 pairs within 10 years.
As it stands, the bald eagles prefer fish — including carcasses tossed back into the water by sport fishermen — and common seabirds such as cormorants and Western gulls. There is no evidence that the eagles currently prey on the islands’ estimated 1,000 cat-size foxes.
But that could change, according to Seth D. Newsome, a researcher with the department of zoology and physiology at the University of Wyoming.
Chemical signatures in bone fragments and feathers found in historic and prehistoric nesting sites revealed that bald eagles often switched diets in order to adapt to dramatic changes in the island’s ecosystem, such as the arrival of ranching operations in the 19th century.
Prehistoric populations preferred seabirds and fish, according to the study. In the late 1800s, the growth of ranching operations resulted in widespread erosion and a decrease in seabird habitat. Bald eagles adapted, Newsome said, by adding the carrion of domestic animals to their diets.
“Our data has raised some warning flags because the maximum number of bald eagles the islands support is currently not known,” Newsome said. “Yet marine resources bald eagles once relied on are not as abundant as they were historically. If their prey base decreases, they could actually switch prey again and start eating island foxes.”
Although the study determined that some ancient bald eagle nests contained the remains of foxes, Newsome said, “we don’t know if those foxes were killed by bald eagles or found dead on the landscape.”
“All we’re saying is that it is not out of the question for a predator of this size to switch prey, and we ought to be mindful of that,” he said.
In the meantime, “our island foxes and bald eagles are doing well,” said Peter Sharpe, a biologist with the Institute for Wildlife Studies, which has a National Park Service contract to oversee the restoration. “They’ve coexisted here for thousands of years.”