On many islands, non-native rodents cause havoc on the local wildlife by killing animals that have not evolved a defense against them. But on the Farallon Islands, the effect of the introduced mice is not as direct.
Every fall burrowing owls stop by the Farallones during their fall migration where they find a plentiful food source in house mice that were brought to the island by humans. Seal hunters, commercial egg collectors, lighthouse keepers and the U.S. Navy and Coast Guard have all occupied the island in the past hundred plus years.
A recent sample estimated there could be up to 1300 mice per hectare during certain times of the year. It is among the highest mouse densities reported in science. This large amount of available food keeps the burrowing owls on the island long after they would normally depart for their winter ranges. In fact, there was still at least one burrowing owl on the island in the middle of April.
But every winter, the mouse population crashes during its annual cycle. The burrowing owls then turn to hunting rare ashy storm-petrels, which begin arriving in late winter in preparation for the spring breeding season.
Ashy storm-petrels only breed on islands and rocks off the coast of California and northern Baja California, including Southeast Farallon Island. It is estimated that there are only about 8000 of these birds breeding each year, half of which are on the Farallones. The U.S. Fish & Wildlife Service has identified the ashy storm-petrel as a Species of Management Concern and the California Department of Fish and Wildlife have listed them as a Bird Species of Special Concern. The ashy storm-petrel is also listed as Endangered by the International Union for the Conservation of Nature.
The US Fish and Wildlife Service and Point Blue Conservation Science are developing a plan to eradicate the mice from Southeast Farallon Island. It is their hope that eliminating the mice will prevent burrowing owls from overstaying into the winter and feeding on ashy storm-petrels. Additionally, removing mice would also help other species such as the Farallon arboreal salamander, invertebrates and plants that would benefit from less predation and consumption.
photos: (mouse) Michael Macor, The Chronicle; (burrowing owl) MONGO; (ashy storm-petrel) Duncan Wright