Farewell to Mice on the Farallon Islands? An Eradication Plan, to Save Storm-petrels

The South Farallon Islands. Photo: NOAA

An inspiring, but perhaps controversial proposal has surfaced off the coast of Northern California: A plan by the Fish and Wildlife Service, and other conservation organizations, to eradicate non-native mice on the South Farallon Islands, which will help a small, spritely, nocturnal seabird—the ashy storm-petrel.

Situated about 30 miles east of San Francisco at the biologically-rich edge of the continental shelf, the Farallones are a jagged cluster of rocky islands and seastacks that host not only a robust population of great white sharks, but fully 25 percent of the California’s breeding seabirds: 300,000 individuals of 13 species. (An incredible number, yes, but before human disturbances, a million birds are thought to have nested there each year.) Among those, the storm-petrel is of greatest concern. Only 5,000 or so exist worldwide, 50 percent of them at the Farallones. But from 1972 to 1992, the number of storm-petrels there has declined by 40 percent.

Ashy Storm-petrel. Photo: Duncan Wright, GNU Free Documentation License

How are mice involved? How, in fact, did they get out there to begin with? Well, it’s thought they were introduced in mid-1800s by seal hunters, commercial egg collectors (what a bonanza it must have been!), and the U.S. Navy, and they’re a problem in their own right, because they compete with the Farallon arboreal salamander for food, eat Farallon camel crickets (great names, right?), and prey on storm-petrel eggs. But the story is more complicated, because each year the Farallones receive a large number of errant landbirds, from hawks to warblers, which use these rocky outposts to rest, often during stormy weather. Among these visitors are burrowing owls, which discover a hearty population of mice to eat. Why not stay, then? They do, and there’s the rub: When mice naturally become harder to find in wintertime, the owls turn to the small storm-petrels.

The islands' managers have tried relocating burrowing owls, enhancing storm-petrel breeding habitat, and using some forms of mice control, but more comprehensive action seems warranted. Get rid of the mice, and the owls would be forced to leave. Thus Fish and Wildlife is thinking of aerially dropping successive waves of rodenticide, in pellet form. Similar rodent removal projects have taken place successfully at 320 islands worldwide. It wouldn’t affect the seabirds, and there aren’t any other mammals on the islands; but it could have an adverse effect on burrowing owls, gulls, and ravens, which worries some. So the plan’s currently up for debate. In April, Fish and Wildlife and the San Francisco Bay Wildlife Refuge Complex published a Notice of Intent to prepare an Environmental Impact Statement, which means the project’s still in its earliest stages. Stay tuned for more.