Northern spotted owls in California are being expelled from their homes by the northern barred owl. To save the state's spotted owls, biologists have recommended removing the invaders.

Northern spotted owls, native to British Columbia, southern Oregon and California's Sierra Nevada, are on the decline. Though they're currently only listed as "near threatened" by the International Union of Concerned Scientists' Red List and the Endangered Species List, biologists worry the northern spotted owl, one of three spotted owl species, could become extant from California as the barred owl continues to expand its range.

Traditionally, the barred owl has been relegated to the Eastern United States, as well as Southern Canada. But in recent years, the species has continued to expand its range into the Pacific Northwest and parts of California.

To find out whether barred owls are encroaching on territory vital to California's spotted owls, researchers deployed a passive acoustic monitoring network across 2,300 square miles in Northern California. The research area comprised acreage in two national forests in the Sierra Nevada.

Owl calls recorded by the audio devices revealed a 2.6-fold increase in the amount of territory occupied by northern barred owls between 2017 and 2018. Scientists determined the barred owls were more likely to colonize old forest habitat favored by spotted owls.

Previous research suggests invasive species often persist at low densities across newly colonized territory for a few generations. This limited invasion is typically followed by a rapid increase in population numbers. Wildlife managers typically wait until this "growth phase" to beef up conservation efforts. By then, it is often too late to reverse the invasion.

The latest study, published this week in the journal The Condor, suggests the invading barred owls of the Sierra Nevada are likely in the early stages of their growth phase. It's not too late to act, authors of the new study contend. But if regulators and wildlife managers don't take the necessary steps, researchers warn, California's spotted owl could find itself without a home in the Golden State.

According to researchers, wildlife managers should err on the side of caution.

"We feel that experimental barred owl removals in the Sierra Nevada are an important step in determining the best long-term management strategy," lead researcher Connor Wood, biologist at the University of Wisconsin, said in a news release.

"This is not something that anyone takes lightly, but we feel that it is warranted because of the very real possibility that continued barred owl population growth could seriously endanger the California spotted owl. By catching this invasion in the early stages, we have a unique but potentially fleeting opportunity to inform policies that could prevent the California spotted owl from being driven extinct in the core of its range," Wood said.

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