Tuesday, March 19, 2013

Channel Island Foxes and Golden Eagles in 2013

NPS photo of a golden eagle in Denali N.P.
Between February and March of 2013, a threat to island foxes on the northern Channel Islands returned - a golden eagle.

Golden eagles migrate north from Mexico and the lower 48 U.S. states up into Canada and Alaska annually. Sometimes this means they fly over the Channel Islands. Historically, golden eagles have not lived on the Channel Islands for two primary reasons:
  1. Golden eagles are mammal specialists. Their chosen diet consists primarily of mammals about the size of a football. The Channel Islands, prior to European settlement, did not have adequate food resources for this large predator to successfully live and breed.
  2. Bald eagles historically thrived on the Channel Islands because they prey on fish, sea birds and carrion. Similar in size, bald eagles are highly territorial and dissuaded the golden eagles,  from spending extended amounts of time in the island ecosystem.

Why Did Golden Eagles Become a Threat to Island Foxes?

Beginning in 1843*, the islands were transformed into ranches. The surrounding ocean was the perfect fence.  Domestic animals (pigs, sheep, goats, rabbits (on Santa Barbara Island) and even cattle) were introduced and then allowed to go wild or feral. Mule deer and elk were also introduced on Santa Rosa Island for paid hunting trips. The young of these species, plus entrails left behind from hunting, attracted golden eagles to the northern islands.

By the mid 1950s bald eagles on became extinct on the Channel Islands because of DDT in the marine ecosystem. Without the bald eagle to drive it away, the golden eagle took up residence on the northern islands. Between 1994 and 2000, golden eagles nearly ate all of the island foxes on San Miguel, Santa Rosa and Santa Cruz Island. Island fox decline.

Positive Conservation Steps

island foxes during captive breeding, NPS
While the island foxes were protected in captive breeding facilities on the islands (2000-2008) conservation efforts helped restore balance on the islands:
  • ~40 golden eagles were captured and relocated to northern California
  • Bald eagles were reintroduced to Santa Cruz and Santa Catalina Islands. In 2006 they began reproducing naturally. The recovering bald eagle population is helping to stop golden eagles from recolonizing  the islands.
  • Feral pigs, sheep and goats were removed and by 2012 introduced deer and elk were removed from Santa Rosa Island

Golden Eagles in 2013

On February 26, 2013 a group of bird watchers spotted a golden eagle at the Saticoy Spreading Ponds in Ventura. During this same time period, two island foxes on San Miguel Island were preyed upon by a golden eagle and a golden eagle was also seen on Santa Rosa Island.

Each year the threat posed by golden eagles flies over the endangered island foxes. Radio tracking collars are a primary means of alerting biologists that a golden eagle has killed an island fox. If predation continues, biologists can respond by trying to locate and capture the lingering golden eagle. Microchips under the skin of each island fox help biologists know the age and life history of individual foxes that are the victims of predation. Friends of the Island Fox raises funds to provide radio tracking collars and microchips for endangered Channel island foxes.

As the Channel Island fox populations recover on the northern islands, golden eagles pose less of a long-term threat. A normal population of 500 or more island foxes can withstand the loss of a few individuals to natural predation by golden eagles passing by. As the natural balance of the islands is restored with dense native vegetation, the territorial bald eagle in residence and robust island fox populations, the occasional golden eagle migrating by will become less of a threat.

*"Channel Islands National Park Timeline", Channel Islands National Park