Mystery of Vanishing Foxes Ends

It was the feather that proved it. The golden eagle shed the tell-tale bronze plume beside a pile of fur on the headlands of San Miguel Island.

For the past five years, scientists have watched helplessly as once robust populations of island foxes have declined on the rugged chain of islands that comprise Channel Islands National Park. Now they have a pretty good idea what is causing their demise. The birds, soaring on long-distance airstrikes from the mainland, are turning baby foxes into eagle chow.

Bird attack has long been suspected as contributing to the decline of island foxes. Golden eagles have been known to swoop down on feral pigs on Santa Cruz Island, and researchers figured that every now and then one would snatch a fox for a side dish. But only recently did they discover just how hard the birds were being on the foxes.

Under an experiment that commenced at the beginning of winter, biologists put radio transmitters on dog collars on eight foxes, including seven pups, on San Miguel Island. Within several weeks, seven of the transmitters fell silent, a bad sign. Astonished, scientists discovered the carcasses of four pups and one adult female fox. All that remained were tufts of distinct gray and cinnamon fur. Of the two remaining foxes, one died of a fur ball and the other of an unknown cause.

“It was unexpected,” said park spokeswoman Carol Spears. “As a result of the radio collaring, we were able to ascertain golden eagles were responsible for nearly all of the pup mortality.”

The discovery goes a long way to explaining why so few pups have survived on San Miguel and Santa Cruz islands in recent years. Scientists have been mystified at the diminishing numbers of juvenile foxes each year.

Although about 450 foxes were found on San Miguel five years ago, just 30 or so remain today. At that rate, island foxes, a threatened species in California, could be eliminated from the island by the end of the year. They are not faring much better on Santa Cruz and Santa Rosa islands, either, though numbers are holding steady on islands farther south, biologists say.

“The mortality of pups is very critical,” Spears said. “If pups are not surviving, we could very well lose the subspecies on those islands.”

Eagle attacks emerge as a serious threat now because other events have depleted fox populations. Human activities at the Channel Islands over the last century have dramatically altered the islands’ rudimentary and sensitive ecosystem and contributed to the demise of foxes.

For example, domestic dogs that boaters bring ashore spread a parasite that clogs a fox’s heart with worms. Cattle grazing stripped away low-lying coastal sage shrub that foxes need for cover. Bald eagles, which eat fish and drive away golden eagles, were eradicated from the islands decades ago because of DDT poisoning. And the introduction of pigs in the 19th century proved enough of a temptation to entice golden eagles to make the 30-mile flight from the mainland, said Kate Faulkner, chief of natural resources for Channel Islands National Park.

Unlike their mainland-based cousins, island foxes lived for centuries with no predators. Consequently, they adopted a risky behavior that many small animals long ago eschewed: venturing out in daylight.

An island fox is no match for a golden eagle. Though they are the largest land mammals on the islands, adult island foxes are about the size of a cat, smaller than those found on the mainland. In contrast, golden eagles are veritable B-52s; with six-foot wingspans, they are the largest eagles in North America.

Confronted with a crisis, the National Park Service is considering drastic action to halt the decline of island foxes. A team of wildlife experts will convene at the Ventura headquarters of Channel Islands National Park April 20-22 to devise strategies to save the park’s foxes.