Invasive barred owl encroaches on California spotted owl in the Sierra Nevada

Bioacoustic researcher records nearly a half a million hours of audio during 2018 looking for barred and spotted owls

A California spotted owl in the Eldorado National Forest - photo: Connor Wood/Cornell Lab of Ornithology

The barred owl is native to the eastern United States, and it’s cousin, the spotted owl is native to the western US. Since the 1980s, western bird watchers and scientists have observed barred owls encroach on the range of the spotted owl in the Pacific Northwest.

Connor Wood has a PhD in Wildlife Ecology and is a post-doctoral researcher at the Cornell Lab of Ornithology in Ithaca, New York. Dr. Wood has studied the shifting populations of barred owls in the Sierra Nevada using novel bioacoustic methods, and on this edition of the Wild Hare, we chat with Connor Wood about his research.

Hear a bioacoustic recording of a barred owl and a spotted owl recorded by Connor Wood/University of Wisconsin-Madison in the Lassen National Forest.

“People were noticing, particularly in the Pacific Northwest, that the barred owl populations growing and that there might be some sort of conflict or competition between the barred owl and the spotted owl,” Wood said in a phone interview.

“And ultimately,” Wood continued. “The people who are predicting that unfortunately were correct. And at this point, the barred owl population in the Pacific Northwest has grown to be very big, and it really poses a very direct and serious threat to the long term future of the spotted owl.”

There are a few sub species of the spotted owl. The northern spotted owl lives in Washington, Oregon and northern California. The California spotted owl is another sub species.

“The core of the California spotted owl’s range is the Sierra Nevada,” Wood said. “And just in the last 10 to 15 years, there started to be anecdotal reports of barred owls now being found in the Sierra Nevada, so this was kind of this early warning that perhaps this invasion of barred owls has progressed from the range of the northern spotted owl down into the Sierra Nevada, the core of the California spotted owl’s range.”

With limited but ominous data, the conservation and management community had a emergent need to better understand how the barred owl invasion is playing out in the Sierra Nevada. According to Connor Wood, that’s necessary for a couple of reasons.

“First is that, a lot of good research has shown just how very clearly that the barred owl poses a direct threat to the spotted owl.

“The other aspect of this is that the Sierra Nevada has a ton of unique plants and animals, and barred owls are pretty flexible and pretty efficient predators. So not only would the arrival of the barred owl in the Sierra Nevada be a threat to the California spotted owl, it could also be a threat to a lot of other species and really could result in a lot of change to that ecosystem.”

For Connor Wood’s PhD work at the University of Wisconsin, he decided to tackle the formidable challenge of better understanding the barred owl invasion. He needed to develop a way to survey owl populations across vast, remote landscapes.

For this, Wood turned to bioacoustics – the use of audio recording devices to locate and ultimately map both spotted and barred owls.

“Conventional owl surveys were just not going to work. That became really quickly apparent because the conventional approach is to go out at night and basically hoot like an owl, and owls will generally hoot right back to you. And that’s how you can get a sense of the population status.

“But as you can imagine, that’s a pretty difficult and time consuming process, and so it’s hard to do it at a really big scale. You only can do that in pretty small areas. And to understand the barred owl invasion, we needed to have really broad coverage across the landscape.”

Both spotted and barred owls have a home range of roughly 1,000 acres. Using bioacoustic methods, in 2017, Wood learned that spotted owls inhabited 45 percent of locations surveyed, and barred owls were found in 8 percent of areas recorded.

A bioacoustic audio recorder in the Sierra Nevada – photo: Connor Wood/Cornell Lab of Ornithology

The assessment was that the spotted owl was well-established, and the barred owl was present but scattered. In 2018, Wood and his team collected 56 years’ worth of audio data in the Sierra Nevada.

“Just one year later (in 2018), the barred owl population had jumped to over 20 percent of our survey locations, whereas the spotted owl had stayed stable and about 45 percent again.

“So this tells us a couple things. It first kind of rules out the possibility of some kind of crazy owl population change, because our spotted owl population was stable. So it’s not that our survey method changed, or we did something differently. It’s a 2.6 fold increase in barred owls in just a single year. And that is what really was alarming. That was the scariest finding. That the barred owl population is potentially entering this phase of really rapid growth.

“And that’s exactly what has happened in the Pacific Northwest, where for over a decade, there’s just a few barred owls here and there, very low numbers, and then within the space of a few years, the population grew really rapidly and pretty much saturated the landscape.”

Data shows, California spotted owl populations were already in slow decline. Wood says what researchers learned in Washington and Oregon is a troubling development for the northern and California spotted owls.

“We see from the Pacific Northwest that the barred owl can cause pretty swift and steep declines in spotted owl populations. So if our spotted owl populations are already declining slowly, that means it’s already at risk. And so if we allowed the barred owl population to really increase, it could really be the end of the California spotted owl because they’re already subject to this variety of ongoing stressors. And so you add one more, and that could really be a severe problem.”

Climate change is affecting the traditional ranges of plants and animals throughout the Sierra Nevada. Spotted owls are a cold-adapted species, and according to Dr. Wood, the summertime high temperatures in the Sierra are already fairly close to what a spotted owl can tolerate. Wood says habitat preservation and working to regenerate big, old growth trees can help mitigate the effects of climate change on spotted owl populations and other species.

“Historically, in the scale of thousands of years, the Sierra Nevada had gigantic old trees. And a huge old tree in some ways creates its own little climate. So if you are an owl or a flying squirrel or any other kind of species, if you can tuck yourself into the shady little crevice on the north side of the 500 year old Douglas fir, it’s going to be a lot cooler there than 15 feet away on a different tree.”

Looking forward, Connor Wood is working on a broader study of the nearly 270 species of bird found in the Sierra Nevada, and the applications of the bioacoustic method are many. For instance, using bioacoustics, researchers can study a forest owl food source, tree frogs.

“People have found that barred owls are eating a ton of tree frogs in the Pacific Northwest. So we could actually study that in the Sierra Nevada, because we can go back to our audio data at sites where there’s barred owls present and scan them for tree frog vocalizations and then see if we find fewer tree frogs at sites with barred owls compared to without barred owls.”

But what to do about invasive barred owls in California spotted owl territory? In the Pacific Northwest, wildlife managers have been experimentally using lethal means to remove barred owls and document the effects on spotted owl populations.

“Experimental removals of barred owls is really a last resort.  And that is something that’s been occurring in the Pacific Northwest where researchers are using lethal means to remove barred owls and tracking whether or not they see the spotted owl population increase in response to the lethal removal of barred owls.

“As you can imagine, no one wants to be doing a lethal removal. That’s absolutely the last option – a last resort  – that anyone is doing. But we have now, a little over a decade of data from the Pacific Northwest showing both the threat of the barred owl to the spotted owl, and now, early evidence that these lethal removals in the Pacific Northwest can be an effective tool that the spotted owl populations may actually respond positively to the removal of barred owls.”

How the barred owl invasion plays out in the Sierra Nevada remains to be seen, but the use of bioacoustics will help more than the spotted owl. Comparing the sound of a forest before and after a fire can help wildlife managers and scientists understand the effects of fire on biodiversity.

“When a fire happens, and they inevitably do, we already have data from before that fire and then we have some post fire data as well. So that can be a way of helping the management communities to understand the effects of fires on biodiversity and also to help them design forest management techniques that can reduce the risk of fires to human lives and property, without compromising biodiversity as well.”


Music Credits as reported through the Public Radio Exchange, in order of appearance

Song: Happy Cycling
Artist: Boards of Canada
Album: Music Has the Right to Children
Label: Warp
Year: 1998
Duration: :042

Song: Clairvoyance
Artist: Jon Hassell
Album: Last Night the Moon Came Dropping its Clothes in the Street
Label: ECM
Year: 2008
Duration: 1:04

Song: Signals
Artist: Seefeel
Album: Quique
Label: Too Pure
Year: 1992
Duration: 2:27

Song: Dream (1948)
Artist: John Cage
Album: In a Landscape
Label: Catalyst
Year: 1948
Duration: 8:33

Song: Ketto
Artist: Bonobo
Album: Days to Come
Label: Ninja Tune
Year: 2006
Duration: 0:40


Brian Bahouth is the editor of the Sierra Nevada Ally and a career public media producer. Support his work in the Ally.