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Competition in the Bull Run: The Spotted Owl's Struggle with an Invasive Relative

Blog Post
Barred and spotted owls are in an unequal match-up as they vie for the same land, food, and nesting habitat to raise their young. And the invasive barred owls are winning, thanks to a few important advantages.

Spotted owls were common in our region’s formerly abundant old-growth habitat, free from the competition posed by barred owls. Historically, dozens of pairs of spotted owls called the Bull Run Watershed home.

And then the barred owl arrived.

Map of united state showing the habitat of the barred owl
Barred owl migration patterns. Courtesy National Audubon Society

Prior to expanding their range, barred owls were confined to the eastern United States. But around 100 years ago they began expanding northward into eastern Canada, then turned west, then south, expanding from Canada into Washington State and then into Oregon and California. By the 1980s, barred owls were being found in the Bull Run.

Barred owls are formidable competitors. They have the advantage of using all forest types, unlike the spotted owl, which strongly prefers old-growth forest, especially for nesting. Barred owls are also aggressive and will physically attack spotted owls, which drives spotted owls into seclusion.

Today, barred owls are probably the single biggest threat to spotted owl persistence in the Bull Run.

“We don’t know how many spotted owls currently occupy the Bull Run,” says John Deshler, Water Bureau wildlife biologist, “But no owl researcher would refute that it is fewer than were historically there.”

A small success

Spotted owls are now found only in the best old-growth habitats in increasingly isolated refuges, where they continue to compete with barred owls that also occupy these premium habitats.

But there have been recent successes for spotted owls in Bull Run.

Surveys in a few areas of the Bull Run, associated with road improvement projects, located two spotted owl pairs and two other territorial males in 2019. Those two pairs nested successfully, producing three owlets among them.

an owl in a tree

But the time for celebration hasn’t arrived just yet. The future of spotted owls in Bull Run remains uncertain.

“Even though spotted owls can sometimes breed successfully in the Bull Run, it is unknown whether their offspring can compete with barred owls and establish their own territories,” says Deshler.

Protecting the Bull Run Watershed

The Bull Run Watershed is the primary drinking water source for the City of Portland. Responsible management, stewardship, and protection of the Bull Run and the wildlife that live there are vital to ensuring the safety and abundance of our region’s drinking water.

Learn more about how protecting and caring for the Bull Run Watershed supports our mission of serving excellent water every minute of every day.