Island Fox Origins and the Truth Behind Eagle Diets

Are you looking for information on the origin of island foxes; how they evolved and their biology? Perhaps you’ve heard people question whether golden eagles were actually preying on island foxes. Answer your questions with information from the primary sources.

Friends of the Island Fox is honored to make information available from top researchers and biologists working with the island fox and the Channel Island ecosystem.


Paul W. Collins, Curator of Vertebrate Zoology at the Santa Barbara Museum of Natural History recently gave two presentations from his accumulated research at an Island Fox Workshop sponsored by the Santa Barbara Museum, Ty Warner Sea Center, Santa Barbara Zoo and Friends of the Island Fox. Both of these presentations are now available through links from the Friends of the Island Fox Educational Research Library.

• Origin, Evolution and Biology of the Island Fox - looks at genetic, morphological, and archeological data regarding island fox origins and the basics of island fox biology, size, behavior, reproduction and diet

• Diet of Bald and Golden Eagles on the Channel Islands - looks at the role eagles played in the decline of island foxes on the Northern Channel Islands and compares the diet of bald and golden eagles on the islands by examining prey remains in nests.

These two slideshow presentations can be found in the Educational Research Library

Under: “Island Fox Fact Sheets & Current Research”

Under: “Links to Research Sources”

Friends of the Island Fox is endeavoring to create the Internet’s most current library of information regarding island foxes. If you are a researcher or biologist and would like us to link to or host your published work relating to the island fox or the Channel Island ecosystem, please contact the Friends of the Island Fox Webmaster at islandfoxnews@gmail.com

Six Islands, Six Different Island Foxes

Island foxes are found on six of the eight Channel Islands off the coast of southern California. (Where does the island fox live? What is an island fox? Experiencing Santa Cruz Island )

A pair of San Clemente island foxes at the Santa Barbara Zoo. (Where can you see an island fox?)


Island foxes all have similar lifestyles and habits:

• About the island fox
• Males consistently weigh more than females
• What do island foxes eat? Their favorite food is the deer mouse. (Each island has its own subspecies of deer mouse.)
• Island foxes are good climbers, which allows them to reach bird nests and eggs
• Island fox pups are born in the spring. Island foxes usually have 2-3, but may have as many as 5 pups

But the foxes on each island are slightly different from each other. The differences are great enough that each island has its own subspecies of island fox.

National Park Service biologist Tim Coonan points out that the San Miguel island fox weighs significantly more than the island foxes on the neighboring island of Santa Rosa.

A San Miguel island fox being fitted for a radio collar.

A pair of Santa Rosa island foxes. (Experiencing Santa Rosa Island)













Tail length is a specific physical trait that varies between the different subspecies. Studies show island foxes from different islands have more or less tail vertebra.

In descending order:

1. San Nicolas Island 22 tail vertebra
   
2. Santa Catalina Island 21 tail vertebra
3. San Clemente Island 19 tail vertebra
4. Santa Rosa Island 19 tail vertebra
5. Santa Cruz Island 19 tail vertebra
6. San Miguel Island 15 tail vertebra

Between the shortest-tailed island foxes on San Miguel Island and the longest tailed on San Nicolas Island there is a difference of 7 vertebra. Interestingly, Schoenherr, Feldmeth and Emerson point out in Natural History of the Islands of California (Univ. of California Press 1999) that the island foxes on San Nicolas Island appear to have the least amount of genetic diversity. This suggests the San Nicolas population may be descended from as few as a single pair of foxes.


All of the island fox populations are vulnerable because they live in such limited habitats. Small population numbers mean that a single introduction of a disease, like the canine distemper outbreak on Santa Catalina Island, can threaten an entire island fox population. (Santa Catalina island fox Update) Catastrophic events, like the arrival of an unexpected predator–the golden eagle–can cause near extinction.

The island foxes on San Miguel Island were reduced to only 15 individuals because of predation by golden eagles. This genetic bottleneck means future San Miguel Island foxes will be more genetically alike than they were in the past.

Conservation and research efforts are necessary across the islands to preserve and protect the six subspecies of island foxes.

Donations to Friends of the Island Fox supports conservation and education efforts to Help Save all six subspecies of island fox. See how your donations can make a difference.

Island Fox Research Notes – Hilary Swarts

Hilary in the field on Santa Cruz Island with one of the oldest collared females, A3Y. (Photo by Jessica Sanchez)

“You can kind of see her collar ID colors: Pink, White, Pink, Pink. That's how we identify the foxes photographed by remote cameras. Annoyingly, it doesn't work all the time. This female just got her newly painted collar in mid October.” - Hilary

Research: Circadian temporal activity of the Santa Cruz Island Fox (Urocyon littoralis): A possible newfound anti-predator defense for a naïve, endangered species

Hilary Swarts, a PhD candidate at the University of California at Davis, is studying the behavioral response of the island fox to golden eagles, a predator to which foxes are not adapted. Her theory is that foxes, which are naturally active during the day (diurnal), have possibly changed their behavior in response to predation pressures from golden eagles. Eagles also hunt during the day, so foxes may be responding by being more active at night (nocturnal). Specifically, the research is investigating whether behavior patterns of foxes on Santa Cruz Island have changed in the period since the 1990’s when golden eagles colonized that island. If foxes have changed their behavior, this shift may be a selection- or learning-based response to predation by golden eagles, or it may be a response to reduced fox density.

Hilary’s research will examine several aspects of island fox ecology including reproductive success, changes in population abundance in response to different levels of predation, daytime activity levels in the absence of predation, and will compare measures of daytime activity to the risk of predation. The results from her research will contribute scientifically to the understanding of the effects of invasions on behavioral ecology, while also providing crucial information to resource managers on the risks of golden eagle predation to the Santa Cruz Island fox population.

Hilary Swarts , Ph.D. Candidate
Department of Wildlife, Fish, and Conservation Biology
University of California at Davis


FIF plans to fund future island fox research. You can help with your donation to Friends of the Island Fox, Inc. To donate use the PayPal or Network for Good buttons in the upper right corner.

For more on Santa Cruz Island and the island fox:

• Experiencing Santa Cruz Island
  
• Where Does the Island Fox Live
• What is an Island Fox
• What Do Island Foxes Eat?