SAN FRANCISCO » A biologist and contractor with a lumber company is at the heart of an experiment sanctioned by the U.S. Fish and Wildlife Service: killing bully owls to protect endangered owls.

Biologist Lowell Diller, a contractor for Green Diamond Resource Co., a lumber company managing timberland in Humboldt, Del Norte and Trinity counties, kills barred owls, which are known to bully the smaller northern spotted owl, the San Jose Mercury News reported.

The barred owl has invaded California from the eastern United States, muscling out northern spotted owls upstate, and spreading south toward San Francisco. A Pacific Northwest native, the spotted owl is threatened with extinction and has become the symbol of the region’s timber conservation battles.

Northern spotted owl populations have fallen in some areas by about 12 percent each year, despite efforts enacted in the 1990s to protect their old-growth forest habitat.

After Diller learned Jack Dumbacher, ornithology curator at the California Academy of Sciences, had a permit to collect some barred owl specimens, Diller saw an opportunity and applied for his own permit.

In 2009, he set aside patches of timberland to remove barred owls. In other patches, he did nothing. After four years, he found that in the areas without barred owls, northern spotted owls are no longer declining.

A study soon to be published in the Journal of Wildlife Management and Wildlife Monographs shows Diller’s grisly conservation experiment works because without barred owls competing for habitat, northern spotted owls bounce back.

But the ethics of the approach leave some conservationists uncomfortable.

“It’s sort of a no-win situation,” said Andrea Jones, the National Audubon Society’s California director of bird conservation. “We’re not advocating for the killing or against the killing.”

She says old-growth habitat destruction is to blame for pitting the two owls against each other.

Shawn Cantrell, Defenders of Wildlife’s northwestern program director, suggested that barred owl removal should play a short-term role in spotted owl conservation, while habitat restoration should play a bigger one. “When we mess things up, we have an obligation to fix them,” he said.

Diller, too, says he doesn’t enjoy killing the animals and tries to focus on what he’s saving: the northern spotted owl. He talks about a pair of owls that emerged from hiding only two weeks after he’d removed barred owls from their old home.

“They were looking for a free mouse and flew up to greet me,” he said. “That was thrilling.”