Of Mice and Birds

By Jesse Irwin
US Fish and Wildlife Service

Published: May 2007

Ashy storm petrel. Credit: Jeff Poklen

Fall on the Farallon Islands is well known for bird migration and feeding white sharks.  For people living on the island, it is also known as mouse season, when mice scurry under foot and every night is another opportunity for one to run across your head while you sleep.  Inconvenience and human sanitation, however, are not the reasons the US Fish and Wildlife Service has proposed eradicating mice from the South Farallon Islands. 

The Farallon Islands are part of the Farallon National Wildlife Refuge managed by USFWS to protect and enhance the wildlife that depends on Refuge lands to breed and rest.  It is the intent of the service to provide the best habitat conditions possible for seabirds and pinnipeds.  Refuge management has been attempting to control non-native plants since the 1980’s with some successes and some failures.  House mice are the last remaining non-native mammal (other than humans); cats and rabbits were removed in the 1970’s.  The mice were introduced early in the sequence of human inhabitation.

The mice are contributing to the decline of ashy storm-petrels by indirect predation and impacting island vegetation to an unknown extent.  Ashy storm petrels are a species of special concern with an estimated worldwide population of 7,000-10,000 individuals.  They have been in a long term decline.  The South Farallon Islands are home to about half the world’s breeding population each summer.  With such a small population and limited distribution, each adult bird predated bring these petrels closer to requiring listing under the Endangered Species Act. 

The abundance of mice in the fall is met by migrating burrowing owls who happily feed on mice before most of them move on.  The few that stay to continue feeding on mice and over-winter, eventually find the hordes of mice in the fall are reduced to a few scattered individuals.  The owls are forced to find an alternate food source. 

The ashy storm-petrels are arriving in large numbers to prospect and maintain nest sites at the same time mice are declining.  That is when biologists begin finding piles of petrel wings under owl roosts, up to several hundred individuals a year.  Owl pellet analysis confirms the seasonal switch in diet.  Owls arriving to find no good food source after the mice are removed will continue migration rather than over-winter and bring the island closer to the natural balance.

The project was first proposed by Island Conservation in 2002 when it drafted a proposal to implement the project.  The National Fish and Wildlife Foundation provided funding for environmental planning and permitting and we expect the implementation to be funded by the Luckenbach Trustee Council.  A public meeting was held in May 2006 to help guide the NEPA process and the public will have another chance for input when the Environmental Assessment is released in late 2007/ early 2008. 

To learn more about the Refuge and read the latest reports click here.

 

Photo Credits: Burrowing Owl: NASA.  Farallones sunset: NOAA