Using the past to peer into the future, scientists warn that a growing population of bald eagles on California's northern Channel Islands might start preying on the rare foxes and seabirds making a comeback there.
The scientists looked at the diet of bald eagles over the millennia, analyzing 40,000-year-old fossil bones and an eagle nest well over 100 years old and filled with 10,000 bone fragments. They concluded that the eagles have historically switched diets to adapt to changing conditions on the islands and might do so again, with unknown consequences for other native species in recovery.
The study, published this month by the online Proceedings of the National Academy of Sciences, is among the first to reconstruct the diet of a bird of prey through prehistoric times, and it contained some surprises.
By looking at carbon and nitrogen isotopes, or chemical "signatures" of ancient bones, the scientists found that tens of thousands of years ago, bald eagles on the islands ate more seabirds than fish and rarely preyed on land or marine mammals. Yet between 1850 and 1950, when the islands were given over to sheep grazing, the eagles' diet abruptly changed. During the lambing season, the birds would gorge on the carcasses of lambs, forgetting seabirds and fish almost entirely.
The lambing season on the islands coincided with the eagles' nesting season, so it was convenient for eagles to pick up the carcass of a stillborn lamb, said Paul Collins, a co-author of the study and the curator of vertebrate zoology at the Santa Barbara Museum of Natural History.
"You have to put yourself in the frame of mind of a predator," he said. "You're not going to want to expend huge amounts of energy going after something that is hard to get."
Historically, sheep overran the islands, trampling them into virtual sand dunes and destroying some of the nesting and roosting grounds of native seabirds. But the carcass of an adult sheep is apparently a tall order even for a scavenging eagle. The study showed that every year, the eagles returned to a diet of seabirds and fish after their chicks fledged.
By the late 1960s, the eagles themselves had disappeared from the islands, victims of egg hunters, game hunters and DDT. They were reintroduced in 2002 by the National Park Service, and now between 30 and 35 eagles make their home there.
As their numbers grow, what will they choose to eat? The last remaining sheep were taken off the islands a decade ago. Fishing stocks near shore are in decline. The seabirds are in recovery mode, and so are the cat-sized island foxes, an endangered species battling back from the brink of extinction.
Satellite tracking data shows that many bald eagles frequent Anacapa Island during the breeding season of native pelicans and gulls. As for the foxes, the eagles have not been much interested, the study showed. In the historic eagle nest, scientists identified the bones of only two island foxes, more than likely scavenged, not killed, by the birds.
But, as previously reported by Miller-McCune, scientists suspect that the bald eagles on the islands today might very occasionally snack on foxes. The park has documented at least 14 fox "kills" by eagles since 2006. That's out of a population of more than 1,000 foxes now living on the islands.
Park officials believe the dead foxes were likely killed by golden eagles from the mainland. Bald eagle feathers have been found in two fox carcasses on Santa Rosa Island, but, again, the foxes could have been scavenged, not killed, by bald eagles.
Finally, there has been an unprecedented explosion in the island populations of California sea lions and elephant seals. These marine mammals are known to accumulate harmful concentrations of DDT, the same pesticide that hastened the decline of bald eagle populations in the first half of the 20th century. There's no evidence in the record that the eagles prey on sea lions, but, as the study notes, the sea lion pupping season coincides with the bald eagle nesting season.
The next step, Collins said, is to analyze the current eagle nests on the island to see exactly what the birds are eating.
"You really have to pay careful attention to see which species are being hit by foraging," he said. "It's important to have an understanding of what the eagles are feeding on. It has ramifications across all these conservation efforts."