Pot smokers, beware: The northern spotted owl, the bird that brought down the Pacific Northwest timber industry, is circling California’s newly legalized marijuana market.

A study led by University of California Davis researchers and released Thursday found that two owl species—the northern spotted owl and barred owl—are being exposed to high levels of rat poison from illegal marijuana grows in northern California.

The problem is expected intensify in the wake of California’s Proposition 64, passed in 2016, which legalized recreation marijuana for adults 21 and over and went into effect Jan. 1.

“Proposition 64, which legalizes recreational marijuana in the state, took effect this month,” said the UC Davis press release. “With its arrival, resource managers expect the number and size of unpermitted, private cultivation sites to grow, which could exacerbate the problem.”

The study, published in the journal Avian Conservation and Ecology, found that “seven of the 10 northern spotted owls collected tested positive for rat poison, while 40 percent of 84 barred owls collected also tested positive for the poison.”

The Fish and Wildlife Service, which funded the study, listed the northern spotted owl as threatened in 1990, which led to millions of acres of federal forest being cordoned off from logging in Washington, Oregon and northern California.

Years later, researchers blamed the arrival of the larger and more aggressive barred owl for the northern spotted owl’s decline, leading the FWS to launch in 2014 a four-year experiment to kill off as many as 3,600 barred owls in the Pacific Northwest.

In the latest study, researchers found that “[e]nvironmental contamination, when coupled with ongoing competition from barred owls, poses an additional stressor on northern spotted owls.”

The research focused on Humboldt, Mendocino and Del Norte counties, areas that have seen their private timberland sold off and converted to thousands of illegal and unregulated marijuana farms.

Those marijuana grows use poison to control rodents, the primary food source of the owls. Previous research by UC Davis faculty member Mourad Gabriel found that fishers, a weasel-like mammal, were being wiped out by the rat poison.

In Humboldt County, researchers estimated the number of marijuana farms at 4,500-15,000, even though the county has received permit applications for a small fraction.

“When you have thousands of unpermitted grows and only a handful of biologists that regulate that for multiple counties, we’re deeply concerned that there aren’t sufficient conservation protective measures in place,” Mr. Gabriel said. “If no one is investigating the level at which private marijuana cultivators are placing chemicals out there, the fragmented forest landscapes created by these sites can serve as source points of exposure for owls and other wildlife.”

Mr. Gabriel was the lead author of the latest study, which was funded by the Fish and Wildlife Service and appeared in the journal Avian Conservation and Ecology and conducted in conjunction with the California Academy of Sciences.

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