Great Gray Owl. Photo: Melissa Groo

Science

After 100 Years, Scientists Are Finally Starting to Understand the Mysterious Great Gray Owl

The elusive owls live in California's Sierra Nevada and have fascinated researchers for a century, but studying them has never been easy.

California naturalist Joseph Grinnell brought down the second owl with a single shot. The first had not been so easy.

It was the evening of June 18, 1915, and Grinnell had been working with a team of field assistants in Yosemite National Park to trap, shoot, catch, and catalog every creature they could find for the University of California Museum of Vertebrate Zoology. One of the men, Charles Holliger, had spotted the owls that morning near Ostrander Rocks, a knobby ridge of granite protruding out of the thick forest. But the owls—species yet unknown—proved uncooperative.

As Grinnell and Holliger worked their way through a “fine forest of fir,” the anxious call of a Ruby-crowned Kinglet cut the evening air. Following the sound, the men flushed a large owl, which landed atop a Jeffrey pine. Grinnell snuck up on the bird and fired—a hit, but no kill. “A Great Gray Owl,” he wrote in his field journal, underscoring the words in surprise. Its “deep reverberating who’s” drew a second owl. This time, Grinnell’s shot was sure.

What would become Specimen MVZ:Bird:25535 was tied up alive and brought back to camp to be photographed as one of Yosemite’s first two Great Gray Owl records. Grinnell surmised the owls were a mated pair; the owl shot second, Specimen MVZ:Bird:25534, was a male. Grinnell noted that the female had a large bare area on her belly and thighs, an “indubitable indication of breeding.”

Joseph Grinnell, the first director of the University of California Museum of Vertebrate Zoology, prepares a field specimen. Photo: The Bancroft Library/University of California

The owls were just two of a total of 4,354 creatures that Grinnell and his team amassed during their ambitious multi-year survey of the park. Yet he recognized the significance of this particular find.

“The discovery of the Great Gray Owl in the Yosemite section was one of the notable events in our field experience,” he wrote in Animal Life in the Yosemite, the 1924 book describing the survey. “And what was most surprising was the fact that the bird was apparently quite at home and nesting. No previous record of the breeding of this northern species of owl south of Canada is known to us.”

Grinnell’s ease in finding the owls would prove to be sheer luck. The California owls were so infrequently spotted in subsequent decades that biologists even reported on stuffed specimens, like one described in 1943 that “was darkened by smoke” from a stint on a family’s fireplace mantle. Grinnell’s The Distribution of the Birds of California, published posthumously in 1944, offered a terse summary of the owl’s situation: “Numbers small, justifying the term ‘rare.’”

Grinnell collected this female owl, MVZ:Bird:25535, during a 1915 survey of Yosemite National Park. Photo: C.D. Holliger Photo, Museum of Vertebrate Zoology Archives, University of California, Berkeley

Seven decades later, biologists are still trying to glean information about the elusive bird. As a species, they are breathtakingly large, North America’s biggest owl by length, nearly three feet long with a five-foot wingspan. Their populations span the top of the globe, across Europe, Asia, Canada, and Alaska, into the northwestern United States. But the California population has long held particular fascination, and scientists have turned to high-tech tools to expose its secrets once and for all.

With his tousled dirty blonde hair, and a wardrobe that skews heavily toward Carhartt jeans, Joe Medley, 33, has that mildly rumpled look of someone who has spent thousands of hours sitting in the woods, listening for rare birds.

One early evening this past spring, Medley, then a graduate student at the University of California, Davis, homed in on a charred ponderosa pine spattered with what looked like white paint. It sat at the edge of a mid-elevation wet meadow in the 900,000-acre Stanislaus National Forest on the western side of Yosemite National Park. He knew the whitewash, combined with a few scattered oval pellets the size of cigar stubs, was strong evidence that a Great Gray Owl had perched there, listening for prey out in the meadow. Thus he got to work, hooking up two black teacup-size funnels to one of the pine’s broken branches. Each funnel held a sensitive microphone wired to a recorder in a waterproof case, capable of logging the sounds of the meadow for a week or more.

An owl pellet found on the edge of the meadow contains the bones of small mammals. Photo: Jake Stangel

It’s a setup Medley has perfected since beginning his master’s degree in 2009. While working on his thesis, he fine-tuned existing computer software to pluck Great Gray Owl vocalizations out of nearly 64,000 hours of sound he recorded. For his Ph.D., he expanded on that work, examining how the remote devices could help locate Great Gray Owl nests and document nesting behaviors. Typically, Medley would just walk away from the recorders and leave them to their job. But on this night he also demonstrated the standard monitoring technique of broadcasting territorial calls into the forest. This method of eliciting a response from nearby males can be useful, but it also has drawbacks for a bird highly sensitive to disturbances. Achieving a less-invasive yet equally effective alternative technique is what makes Medley’s approach cutting edge.

He reached over to a speaker in a waterproof box on the ground and started broadcasting a taped recording.

Whoooo ... whoooo ... whooo ... whooo ... whooo ... whooo ... whooo ... whoo!” the speaker rumbled. Listen:

Medley sat in silence, his head swiveling slightly, straining to sieve a response out of the soft background chattering of Hermit Warblers, robins, and crickets.

Whoooo ... whoooo ... whooo ... whooo ... whooo ... whooo ... whooo ... whoo! the speaker boomed again. An airplane roared overhead, distant but distracting.

The broadcast tape played out a 10-minute cycle, but nothing answered. Then as Medley was beginning to pack up, he detected a faint, unmistakable hoot of a Great Gray Owl on the opposite edge of the meadow: “Whoooo ... whoooo ... whooo ... whooo ... whooo ... whooo ... whooo ... whoo.”

He froze and a wide smile spread across his face. Bingo. 

Great Gray Owls can be difficult to detect, even for experienced field biologists. Joe Medley has gotten around this problem by using a remote recording system. Photo: Jake Stangel

Historically, ornithologists have had far more rudimentary tools at their disposal. In 1979, Jon Winter, a northern California birder, drove more than 12,300 miles in a blue Nissan pickup, spending 70 days in the field traversing the bird’s presumed range. In spite of his efforts, he located just seven birds and five nests. Ultimately, he estimated the state’s population at 53 birds, a find that led California to list the Great Gray as a state endangered species in 1980.

For the next two decades, only a handful of studies added to that body of knowledge. Then West Nile virus landed in North America in 1999, famously causing crows to drop dead in the streets of New York City, Washington, D.C., and beyond. As the virus moved west, its casualties included all 27 captive Great Gray Owls at The Owl Foundation, a bird-rehabilitation facility in Ontario. John Keane, a wildlife ecologist at the Forest Service Pacific Southwest Research Station, saw opportunity in those grim statistics. 

In 2004, as West Nile virus spread throughout California, Keane drummed up funding to collect blood samples from Great Gray Owls and Spotted Owls to screen for antibodies to the disease. More followed, giving researchers momentum to address other questions: How could they best track Great Gray Owl populations, given that they were so rare and hard to find? And were the Sierra Nevada owls genetically different from the rest of the North American population?

Keane and Josh Hull, now recovery division chief for the U.S. Fish and Wildlife Service’s Sacramento field office and an adjunct professor at University of California, Davis, realized they could use blood samples from the West Nile survey to determine how long the Sierra birds had been separated from the main population. “We knew that [Great Gray Owls] are isolated now, but we didn’t know whether that meant they were isolated a million years ago, 500 years ago, 50 years ago,” Hull recalls. They also decided to collect molted feathers to see whether Hull could extract DNA to identify individual birds.

From the fieldwork’s start, banding Great Gray Owls proved a challenge. The animals are well camouflaged and easily spooked. “If you see a Spotted Owl, you can walk right up to the tree where it’s sitting,” said Medley, who joined the team in 2007. “With Great Gray Owls, they see you from a long ways away and they will flush, and you wouldn’t hear them.” Even after the team managed to catch 32 birds using a baited trap, the bands proved nearly impossible to see among the thick feathers covering the owls’ legs. To truly make progress, they needed new techniques. 

Joe Medley carries recording equipment to set up in a meadow. Photo: Jake Stangel

The world is full of elusive bird species.

To search for some of them, such as the Ivory-billed Woodpecker, Kurt Fristrup, a bioacoustical scientist with the National Park Service’s Natural Sounds Program, had begun using remote recorders. When he encountered Medley and Keane at a Great Gray Owl symposium in 2008, he encouraged the pair to try using them to eavesdrop on the owls. Medley jumped at it.

The next field season, Medley was hanging his recording equipment on the fringes of 50 wet meadows in Yosemite National Park and in the Stanislaus National Forest. Months later he had about 40 terabytes of data, only 5 terabytes shy of the volume of data collected by the Hubble Space Telescope in its first 20 years of observations. His next step was to fine-tune the voice-recognition software to distinguish between other animals and the owls, and then the unique sounds of mom, dad, and their chicks.

This proved tremendously difficult. At first, the software tagged everything that remotely sounded like an owl: Coyotes yipping. The rumble of a high-elevation jet. Bears crunching on the microphones. The constant beeping of a Red-breasted Nuthatch, like a truck stuck in reverse.

The worst were the incessant alarm calls of Douglas’ squirrels. No matter how much Medley tweaked the software, the squirrel calls “skipped right through,” because they were so similar to juvenile owl calls. It was a major problem. If he couldn’t find a way to deal with all the false positives, he wouldn’t be able to use the software to detect young birds, a key indicator of the population’s health.

Meanwhile, results from the blood samples were trickling in—no trace of West Nile virus antibodies. But they did show clear genetic evidence that the Sierra Nevada owls were a separate subspecies. “It was pretty exciting,” Hull says. “We didn’t know it would be as distinct as its own subspecies when we started.” They published the findings in 2010: This tiny population of Sierra Nevada owls, now named Strix nebulosa yosemitensis, had managed to survive despite isolation from populations in Canada for nearly 27,000 years, a relict population left over from when the owls lived in the south during the last Ice Age—a boreal bird that has evolved to cope with a much more temperate climate.

By the following winter, Medley had used a statistical tool called a random forest analysis to finally crack the false-positive problem and make his surveillance technique work at a practical level. As he culled the data, insights began to emerge. For one thing, Great Gray Owls were positively noisy when people weren’t around. In the end, he identified 7,445 male, 13,163 female, and 43,004 juvenile calls, and showed that acoustic monitoring was as effective as traditional survey techniques, with the advantage that it doesn’t bother the birds.

“Say you have a project coming up like a road expansion,” Medley says. “If your goal is just determining whether or not an owl is there, you can use the recorders and it doesn’t disturb the birds whatsoever. You can get comparable results to the more-invasive traditional broadcast methods.”

The recorder logs sounds for a week or more. Photo: Jake Stangel

Park ornithologist Sarah Stock oversees Yosemite’s Great Gray Owl program, which includes funding for Medley’s research. She sees the value of using the remote-recording units in Yosemite, especially when it comes to figuring out where a pair of owls is nesting. “Nest searching for Great Gray Owls is extremely difficult, in contrast to other species of owls,” she says.

But Stock is even more excited about the team’s genetics work. So far, more than a hundred individual owls have been identified from the feathers found in 8 to 10 meadows. “It could be a real game changer,” she says. Currently, if her survey crews see a Great Gray Owl, they have no idea whether it is the male that was there the previous year or a new individual. DNA from dropped feathers can answer that question.

No one knows how much individual birds move around during the summer, or how long the birds live, or whether adult birds return to the same meadows year after year. Those questions can now be answered, too. More important, by tracking the number of individuals, researchers can tell if the population is growing, stable, or in decline—a critical piece of information when the owl numbers are so vanishingly small.

The scientists traverse a wet meadow in the Sierra Nevada where Great Gray Owls prey upon small mammals. Photo: Jake Stangel

Although the core of the California Great Gray Owl population lies within Yosemite National Park, its boundaries can't protect the birds from vehicle collisions, wildfires, disease, and climate change. Just three years ago, the Rim Fire incinerated an area more than six times the size of Washington, D.C., burning 10 of the 18 meadows in Yosemite with suitable nesting habitat for Great Gray Owls. Cattle grazing, the loss of large dead trees for nesting, and accelerating second-home development put additional pressures on birds outside the park. Climate change poses another major threat. Scientists listed the owls in 2014 as one of the bird species in the Sierra Nevada that will be particularly vulnerable to changes from a warming planet.

Given these pressures, a coalition that includes the National Park Service, the Forest Service, the California Department of Fish and Wildlife, and The Institute for Bird Populations is working on projects to increase protection for the owls. A 2015 study cataloged the 56 known nesting records in California since 1973, and found a surprising 21 percent of the nests in hotter, lower habitats—raising a red flag, since this is also prime land for second-home development.

The study also underscores the importance of artificial nest trees, which have been the Forest Service’s answer to the special needs of the Sierra Nevada owls. Unlike their northern cousins, which nest mostly in abandoned stick nests, the Sierra Nevada birds nest only in snags, the broken-off tops of large-diameter dead trees, which can be in short supply given logging practices in the national forests. The lack of suitable nesting trees may explain why there are so few Sierra Nevada owls. Forest managers have tried to remedy this problem by creating artificial snags, lopping off the tops of selected big trees near prime wet meadows.

Medley’s detective work helps address another challenge on Forest Service property. The public lands are “multiple use,” so ranchers can get permits to graze cattle in the lush wet meadows the birds favor. “Grazing—and overgrazing—are a primary threat,” Keane says, because cows can alter meadows by chomping and trampling grasses, which reduces prey numbers. But if biologists know where the birds and their nests are located, they can protect those areas.

The state’s new Great Gray Owl conservation plan, which is still pending final approval, builds on this knowledge. It encourages land managers to safeguard the big-diameter trees the owls depend on, promotes the use of artificial nesting trees where natural ones have been cut or destroyed by fire, and names the feather DNA forensics and Medley’s remote-recording technique as new monitoring options to consider. 

A pan trap used to catch owls for banding. Photo: Jake Stangel

Back in the Stanislaus National Forest Medley followed up his evening of broadcasting calls with a visit to one of his favorite meadows, emerald and radiant after a recent rain. Hermit Warblers and juncos trilled atop a soft chorus of katydids and crickets as Medley pointed out “tree islands,” groups of trees that stick out like thumbs into the grassy areas, a key feature that attracts owls.

The tree islands offer good perches from which Great Gray Owls can listen for and catch voles and gophers, he said, which is one reason he often finds owls here. As if to underscore his words, two Steller’s Jays erupted from across the way, screaming excitedly. Medley stopped, raised his binoculars, and pinpointed the source of their distress: a Great Gray Owl perched on a Jeffrey pine about 100 yards from where he stood. Its fine brown-and-gray feathers neatly blended against the pine’s dappled trunk. The giveaway to the bird’s presence: the thin white “mustache” of feathers underneath its huge facial discs, flashing now and then like a faint smile.

A century ago, Joseph Grinnell was alerted to a different owl on another Jeffrey pine by the scolding chatter of a Ruby-crowned Kinglet, and wondered how a bird from the north could be so at home nesting in the wilds of the Sierra Nevada.

Medley and his colleagues now know the answer to Grinnell’s question. The owl that gazed across the meadow was an Ice Age remnant, a northern bird that proved adaptable enough to survive in tiny numbers in California’s more temperate climate. Now the biologists believe they have the tools to secure the birds’ future here.  

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