Survival of Farallon birds hinges on eradicating islands' mouse population 

A proposal to eradicate tens of thousands of mice from the Farallon Islands could involve dropping a payload of poison pellets, all in the name of preserving a fragile seabird population.

Initial talks of killing the non-native rodents on one of the desolate isles 27 miles off the coast of San Francisco have some environmental groups fuming, while others don’t see a better option.

To see a diagram of the poison pellet plan and its potential effect on other species, click on the photo to the right.

Exterminating the mice — all the mice — is the subject of a draft environmental impact statement expected in the fall from the U.S. Fish and Wildlife Service. Although federal officials say nothing has been finalized on how to deal with the mouse problem, a meeting in May centered on concerns about the poisoning option.

At stake is the survival of the declining Ashy storm-petrel, because according to Audubon California, more than half the species’ remaining 7,500 members rely on the Farallones as a breeding ground. Aside from anecdotal reports of mice eating petrel eggs and chicks, they also attract hungry burrowing owls, which come for the easy rodent feast in the fall and stay for the winter with nothing to eat but the petrels.

That’s because the mice experience an incredible annual boom and bust in numbers. The population peaks in October at more than 30,000, according to Fish and Wildlife estimates, but their numbers dwindle to mere hundreds when the December rains begin, forcing some to survive by way of cannibalism.

Peter Pyle, a biologist formerly with the Point Reyes Bird Observatory who spent 2,100 days of his life on the islands from 1980 to 2003, said if poison is dropped there, it should be in December to kill the entire mouse population when it’s at its weakest, and before the petrels arrive for nesting in February.

Opponents of the idea fear it would kill seagulls that eat the pellets, or burrowing owls — also a species of concern — that eat the poisoned mice.

“We want them to consider other options that will not impact wildlife,” said Maggie Sergio, director of the San Rafael-based nonprofit Wildcare. “This will get into the water and impact aquatics, and this will impact other species.”

Poison also has been used at the Aleutian Islands in Alaska and the Channel Islands off the coast of Southern California to kill invasive rodents that threaten birds.

“We have concerns about rodenticides, but doing nothing is not an alternative,” said Graham Chisholm, executive director of Audubon California. “The storm-petrel is clearly a bird that has special concern here. Almost its entire global range is in California’s waters.”

Doug Cordell, a Fish and Wildlife spokesman, said public comment will be accepted on the draft environmental report in the fall, and the earliest any poisoning would occur is in late 2012.

“One alternative might be to do nothing, if the risks to the ecosystem outweigh the benefits,” Cordell said.

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Dan Schreiber

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