The hidden owl war: An invasive owl is conquering ancient forests, putting the threatened northern spotted owl at even greater risk

An invasive owl species is occupying the same ancient forests that northern spotted owls rely on for survival, according to a new study that used GPS tracking to reveal the extent of the ecological conflict.

Researchers found that barred owls consistently selected the tallest, oldest and most structurally complex forests in northwestern California—habitats with large trees, multilayered canopies, dense vegetation and abundant nesting cavities. These are also the forests most important to the threatened northern spotted owl for nesting and roosting.

Published in Biological Conservation, the study suggests barred owls are not simply expanding into spotted owl territory—they are concentrating in the exact habitats where the native species most needs refuge.

GPS tracking reveals extensive habitat overlap

Scientists from the Integral Ecology Research Center tracked 56 barred owls across nearly 4,070 square kilometres of forest using GPS telemetry, allowing them to study habitat use at the level of individual birds.

The findings showed a clear preference for mature forest structure. On average, 64% of a barred owl's home range overlapped with mapped northern spotted owl habitat, while 38% overlapped with the highest-quality nesting and roosting forests.

The researchers say this overlap indicates that protecting old-growth forests alone may not be enough if an aggressive competing species continues to occupy them.

Why are barred owls a threat?

Barred owls are native to eastern North America but gradually expanded west during the twentieth century as connected forest corridors allowed their range to spread.

Today, they occupy much of the Pacific Northwest and northern California, where northern spotted owls are already under pressure from logging, habitat fragmentation and wildfire. Although both species use similar forest resources, barred owls are more adaptable in their diet and habitat preferences, giving them a competitive advantage.

Their presence creates biological interference, reducing the native owl's access to otherwise suitable habitat.

California barred owls occupy much larger territories

The study also uncovered a striking ecological difference between barred owls in California and those in their native range.

In northwestern California, a barred owl occupied an average home range of 6.7 square kilometres—around 28 times larger than territories previously recorded by the same research team in Louisiana.

While birds do not use every part of their territory equally, the researchers say such large ranges mean each invasive owl can influence a substantial area of forest, affecting the distribution and behaviour of spotted owls across the landscape.

Impact reaches beyond two owl species

The ecological consequences extend beyond the spotted owl.

Barred owls are generalist predators that hunt flying squirrels, tree voles, salamanders, reptiles and other small mammals—many of which are important components of the old-growth ecosystem. They have also been documented preying on species that spotted owls rarely hunt, including western spotted skunks.

The authors caution that the broader effects on forest biodiversity remain incompletely understood and require further research.

Removing barred owls reopened key habitat

One of the study's most encouraging findings came from areas where barred owls had been removed through a permitted research and management programme.

Using records from 1,171 removed barred owls, researchers estimated that more than 1,800 square kilometres of spotted owl habitat had become available again for possible recolonisation. Nearly 1,150 square kilometres of that area consisted of the highest-quality nesting and roosting forest.

The researchers note that reopened habitat does not guarantee immediate recovery, but it demonstrates that predator management can restore access to biologically intact forests.

“Habitat isn’t just trees, it’s trees a spotted owl can actually live in,” said Vitek Jirinec, a research scientist at the Integral Ecology Research Center and lead author of the study. “Managing the invader is proving just as critical as protecting the trees themselves.”

A larger conservation challenge

The findings support ongoing conservation efforts involving the University of Wisconsin–Madison, the U.S. Forest Service, the California Department of Fish and Wildlife, the Hoopa Valley Tribe and other partners. These programmes are studying whether spotted owls return to territories after barred owls are removed.

Although the strategy remains controversial because it involves removing one native North American bird from regions it colonised naturally, wildlife agencies argue it is necessary to prevent the continued decline of the northern spotted owl.

Greta Wengert, co-founder and co-director of the Integral Ecology Research Center, said the issue extends far beyond a single endangered species.

“Spotted owls are the reason this study exists, but they’re part of a much larger old-growth food web,” she said.

The researchers conclude that long-term recovery will depend on two parallel strategies: protecting the remaining ancient forests and managing the invasive barred owl populations that now occupy them.