Technical Report

Annual variation in the diet of house mice (Mus musculus) on southeast Farallon Island

Numerous declines and extinctions of native wildlife populations have been caused by the introduction of non-native plants and animals to islands (Huyser et al. 2000, Marinez-Gomez and Jacobsen 2004). Various members of the Order Rodentia have contributed to this world wide phenomenon (Cuthbert and Hilton 2004). Many rodents are omnivores and members of the taxa have been known to feed on bird eggs, birds, insects, ant eggs, meat, bacon, cannibalism, lizards, carrion, slugs, and mammal bones (Landry 1970). Previous studies found house mice (Mus musculus) diets to be omnivorous but the diet varied with location of the study (primarily invertebrates in cultivated regions of North America, Whitaker 1966; invertebrates, vegetation, and vertebrate material on sub-Antarctic Islands, Copson 1986; and seasonal change with food availability on South Atlantic Islands, Jones et al. 2003). House mice have also been found to damage eggs (Maxon and Oring 1978) and feed on live chicks of Tristan Albatross (Diomedea dabbenena) and Atlantic Petrel (Pterodroma incerta) (Cuthbert and Hilton 2004). Egg size probably does not provide protection against mouse predation as Blight and Ryder (1999) found that Peromyscus consumed the eggs of Rhinoceros Auklets (Cerorhinca monocerata). In addition to direct affects on seabirds through predation, house mice have been found to alter the plant and invertebrate communities on islands and result in greater effects than just through predation (Chown and Smith 1993, Smith et al. 2002). Mice may also indirectly affect seabird populations by supporting winter populations of predators which switch their diet to the seabirds when seabirds become available at nesting colonies in the spring (Drost 1989). Southeast Farallon Island, in the Farallon Island archipelago, is located 32 km southwest of Point Reyes, California, USA. This important seabird breeding colony in western North America, includes the world’s largest populations of breeding Ashy Storm- Petrels (Oceanodoma homochroa), Brant’s Cormorants (Phalacrocorax penicillatus), and Western Gulls (Larus occidentalis) (DeSante and Ainley 1980). Additionally there are 12 other seabird species with populations on the Farallon Islands (DeSante and Ainley 1980). Many of these seabirds are ground-nesting or burrow-nesting species including the Ashy Storm- Petrel, Cassin’s Auklet (Ptychoramphus aleuticus), and Rhinoceros Auklet (Ainley and Boekelheide 1990). Sydeman et al. (1998) reported a 28-44% population decline for seabirds from 1972 to 1992. They speculated that this decline was potentially attributed to weather patterns (El Niño), human disturbance, burrowing owl predation on adults, or predation on eggs and chicks by house mice (Sydeman et al. 1998). House mice were introduced to Southeast Farallon Island by human settlers, likely after the building of a lighthouse in 1855 (Howald et al. 2004, Shorenherr et al. 1999). Few studies have examined the ecology of house mice on the island (McDermott 2002, Schwan 1984). These non-native populations of house mice have the potential to affect survival of Ashy Storm-Petrels and other ground- or burrow-nesting birds by: 1) possibly eating or destroying eggs and reducing breeding success, 2) dispersing seeds of non-native plants species lowering the habitat quality for ground nesting birds, and 3) providing a winter prey source for over-wintering predators which subsequently feed on seabirds when they become available in the spring (Howald et al. 2004). Eradication of the house mouse on Southeast Farallon Island has been proposed (Howald et al. 2004). Successful eradication of non-native rabbits and cats occurred on the island following the creation of the Farallon National Wildlife Refuge in 1969 (Howald et al. 2004). Eradication of rats or mice has successfully occurred on numerous islands around New Zealand, sub-Antarctic islands, the Aleutian Islands, and on Anacapa Island in southern California (reviewed Donlan et al. 2003). The justification for eradication of mice from Southeast Farallon Island could have two supporting goals. The first would be to preserve the community in its natural state without human introduced, non-native species. The second, would be the removal of the mice to help the declining populations of seabirds on the island, but this later goal is based on the presumption that mice negatively affect the seabird populations. The lack of knowledge about seabird population size prior to the introduction of house mice makes studies that demonstrate the impacts of mice on the seabird population and the island community necessary. One part of the assessment of the impact of mice on the island community is to analyze the diet of the mice. Through examination of the material consumed by the mice during the entire year, we could evaluate whether mice have the potential to influence plants, invertebrates, and seabirds on Southeast Farallon Island. This project initially began as a survey of the diet of the mice at the island as part of an undergraduate thesis at Humboldt State University (Chapter 1). The initial survey included the months of February 2002 to March 2003. However, there was no data from April to August 2002 the time period when eggs or chicks could be available for consumption. Subsequent to the first survey, additional mice were acquired from the period April 2003 to August 2004. Which included the time period when eggs or chicks would be available. A second undergraduate thesis (Chapter 2) was under taken that utilized the insight acquired in the first thesis to evaluate the period that seabirds would be vulnerable. We summarize the two theses and provide conclusion in the final section of this report.