Wild Neighbors: Collateral Damage in the Rodent Wars

By Joe Eaton
Tuesday May 10, 2011 - 10:55:00 AM
Ashy storm-petrel in the hand: victim of killer mice?
Duncan Wright (via Wikimedia Images)
Ashy storm-petrel in the hand: victim of killer mice?
Burrowing owl: Fish and Wildlife Service scapebird?
Alan D. Wilson (via Wikimedia images)
Burrowing owl: Fish and Wildlife Service scapebird?

South Farallon Island, that great seabird metropolis 32 miles west of San Francisco, is infested with house mice. That’s nothing new; the mice have been out there since the 1800s. There used to be feral cats and rabbits as well, but they were exterminated around 1974. 

Now the US Fish and Wildlife Service, the agency that manages the Farallones National Wildlife Refuge, is going after the mice. Their plan is to air-drop the potent rodenticide brodifacoum in pelletized form all over South Farallon. FWS says this is necessary to protect the island’s population of ashy storm-petrels, a California Species of Special Concern. This small nocturnal seabird nests only on offshore islands along the California and Baja California coasts, with an estimated 4000 breeding pairs on the Farallons. The Channel Islands have a similar, perhaps slightly larger population. 

I am not by any means a mouse-hugger. I recognize the need to manage, lethally if necessary, invasive animal or plant species that threaten the survival of vulnerable native species. But I don’t think FWS has made a convincing case that aerial broadcast of rodenticide is the best option in this situation. History suggests that it’s difficult to pull off this kind of operation without significant nontarget mortality—the killing of other wildlife species, including some of conservation concern. 

Although rodenticide airdrops were apparently pioneered in New Zealand, the technique was first used in North America in 2001-02, against black rats on Anacapa Island, part of the Channel Islands group. (If you’ve read T. C. Boyle’s new novel When the Killing’s Done, this story may be familiar.) The island is home to an endemic subspecies of rufous-crowned sparrow, a state Species of Special Concern, and an endemic subspecies of deer mouse. FWS set aside a no-drop zone to minimize the impact on the sparrow, but a recent report in the conservation journal Oryx acknowledged significant (unquantified) mortality. The native mice also took a hit; some were taken into protective custody beforehand and later released to repopulate the island. FWS also tried to mitigate raptor mortality by capturing and/or relocating Anacapa’s hawks and owls; nonetheless, 3 barn owls, 6 burrowing owls, and an American kestrel were poisoned. Other victims: 9 species of passerines, from ravens to orange-crowned warblers, and western gulls. 

In 2008, FWS dropped 46 metric tons of brodifacoum bait on Rat Island in the Aleutians to eradicate the resident black rats. Afterward, carcass surveys recovered 46 dead bald eagles; 12 of the 16 tested for brodifacoum had lethal levels. Other casualties: 

320 glaucous-winged gulls (24 of 34 positive) and 25 other species of birds. With the Anacapa experience to draw on, this is not an impressive learning curve. Despite mitigation attempts, non-target mortality seems inevitable. 

Beyond that, the extent of the mouse problem on South Farallon is not entirely clear from the agency documents I’ve seen. Mice do attack petrel eggs and chicks. But some documents, notably the Farallon National Wildlife Refuge Final Comprehensive Conservation Plan and Environmental Assessment ( and its appendices, put more emphasis on predation by wintering burrowing owls. Note that the burrowing owl is also a Species of Special Concern. 

Here’s the scenario: every year, juvenile burrowing owls, an average of 2 to 5 annually, somehow make their way to the islands. They prey on the mice until the mouse population crashes, as it periodically does, in late winter. Then they switch to ashy storm-petrels. By the end of the season, most of the owls have either starved to death or fallen prey to the islands’ peregrine falcons. The mice are important not so much as direct predators as a temporary prey subsidy to the owls. 

FWS invokes the Santa Cruz Island pig-eagle-fox case as a comparable situation. The short version is that bald eagles died out during the DDT years. Golden eagles moved in from the mainland and preyed on the superabundant feral pigs. They also went after the piglet-sized endemic island foxes, driving them to near-extinction. 

You’d think that somewhere in its documents FWS would quantify the relative impact of mice, owls, and western gulls, which are also known to prey on storm-petrels. But no. It’s not clear how many petrels are killed by owls, or how many individual owls do the killing—although the CCP notes that some gulls become specialist petrel predators and notes the option of removing those individuals. If so few owls are involved, why not simply trap and relocate them as soon as they show up—especially since they are unlikely to survive the winter in any case? 

There’s also an interesting report from PRBO Conservation Science, which has long had a presence on South Farallon. It calls into question the assumption that ashy storm-petrel populations are in fact declining. Analysis of mist-net captures of petrels from 1992 to 2010 showed no clear trends. The report mentions gull and owl (but not mouse) predation, but concludes: “Neither overall predation nor predation by species, correlated with CPUE [Catch per Unit Effort.]” You have to wonder if the petrel is in imminent danger of extirpation after all. 

We need to know a great deal more about predator-prey interactions on the Farallons before we resort to saturation-bombing with rat poison. FWS is currently accepting comments on the South Farallon Islands Non-native Mouse Eradication Project. Letters can be addressed to: 

South Farallon Islands NEPA Scoping Comments 

c/o Gerry McChesney 

Farallon NWR Manager 

9500 Thornton Avenue 

Newark CA 94560