Surveys conducted from the 1970s through the early 1990s indicated the island foxes on Santa Cruz Island were doing well. However, starting in 1994 researchers noted a decline in capture success and the concurrent death of animals equipped with telemetry collars. The researchers concluded that the primary cause of death of the foxes was predation by golden eagles. Puncture wounds found in the skulls of dead foxes, as well as the presence of golden eagle feathers at the carcasses helped confirm the cause of death. Foxes completely disappeared from one long-term study area on Santa Cruz Island, and were severely reduced in another area. In 1999, an estimated 133 foxes remained on Santa Cruz Island. Golden eagles had not historically nested on the Channel Islands, but were observed as occasional visitors. It is surmised that golden eagles have more recently been attracted to the northern Channel Islands by the availability of feral pigs and sheep as a food source. It has been further suggested that the disappearance of bald eagles from the Channel Islands during the 1950s may have removed a deterrent to golden eagles. As bald eagles and golden eagles do not typically overlap in their breeding areas, and both species are very territorial, the historic presence of bald eagles likely inhibited occupation of the Channel Islands by golden eagles.
To help mitigate the problem of golden eagle predation on fox populations on the northern Channel Islands, the National Park Service enlisted the services of the Santa Cruz Predatory Bird Research Group (SCPBRG) to capture and relocate eagles from the island. By April 2000, a total of 13 golden eagles had been removed from the island and translocated to other areas in California. However, in 2000 the fox population was continuing to decline, primarily due to eagle depredation. The Institute was contracted by The Nature Conservancy to: 1) determine the demographics of the foxes remaining on Santa Cruz Island, 2) monitor survival and mortality factors using telemetry, 3) establish and maintain a captive breeding facility on the island for island foxes, and 4) provide recommendations for recovering and conserving the fox population. Using techniques we pioneered on Santa Catalina Island, IWS released captive-bred fox pups into the wild to help bolster natural production and assist the population’s recovery. From December 2000 to January 2007 the Institute monitored 205 foxes, of which 65 were captive-born.
During monitoring, 71% of the 73 foxes found dead were killed by golden eagles. Continuing its previous effort, the SCPBRG trapped and removed an additional 18 golden eagles between April 2000 and June 2004. The remaining eagles were extremely wary and were becoming increasingly difficult to trap using conventional techniques. In 2005, IWS took over the golden eagle removal effort and was tasked by the U.S. Fish an d Wildlife Service, the National Park Service and the California Department of Fish and Game to come up with new and innovative techniques to capture the remaining birds. The last pair of breeding golden eagles was trapped and removed in 2006, and no breeding attempts have been observed since that time. Thus far, none of the eagles removed from the island have returned.
Starting in 2002, IWS initiated a program to reestablish bald eagles to Santa Cruz and the rest of the northern Channel Islands, funded by the Montrose Settlements Restoration Program. It was hoped that part of the benefit of this reintroduction effort would be to help reduce the chance that golden eagles would return to attempt to nest on the northern islands if resident bald eagles were present. This effort, in addition to the complete removal of exotic and feral livestock from the island, should make the islands less desirable in the view of golden eagles looking for nesting sites.
In 2007, The Nature Conservancy took over monitoring of the fox population and in 2009-2010 estimated a total population (adults and pups) of 1200 foxes! While golden eagles are still an occasional problem for the foxes, only 2 of the monitored foxes were predated by golden eagles during 2009-2010 on Santa Cruz Island. The Nature Conservancy and Colorado State University are conducting research based on trapping foxes for monitoring, while developing management plans to reduce predation by golden eagles. The Institute continues long-term cooperation with all agencies and organizations to maintain the health of the Santa Cruz Island fox population.