First Look

The Hills Have Ears

New radio towers are bringing a sea-change in wildlife tracking.

February 22, 2024
A Motus tower set up by Audubon Canyon Ranch in Marin County. (Photo courtesy of Audubon Canyon Ranch)

It was the fall of 2022 in Muir Woods when the object of Gabriel Reyes’ long obsession flew right into his trap. 

Reyes and other scientists from the U.S. Geological Survey had set up mist nets with hopes of catching a hoary bat (Lasiurus cinereus), a widespread solitary species that flocks to the Bay in the fall and spring. Every summer, the bats vanished. Where did they go, and how did they get there? 

This time, Reyes finally had a way to keep tabs on his subjects—thanks to a fast-growing global network called Motus that is transforming wildlife tracking.

Reyes untangled the bat, which weighed about an ounce and a half, and carefully snipped the hair between its shoulder blades. On this undignified new bald spot, he glued a piece of nanotech—a button-sized white tag with a long antenna—effectively linking his subject, now ID #43190, to a large and growing network of radio towers.

“There’s a real conservation need to understand these issues,” Reyes says. About 40 percent of the bats killed by wind turbines in North America each year are hoaries—an estimated 128,000 individuals. With better data about their flight patterns, researchers could advise policymakers on how to curb such fatalities. 

Gabriel Reyes holds out a hoary bat that has been fitted with a Motus tracker. The tags weigh 0.01 ounces—around 1 percent of the bat’s weight. (Photo Courtesy U.S. Geological Survey)

The Motus network, launched and coordinated by Birds Canada and named after the Latin word for movement, is driving a sea-change in animal telemetry by automating it. The Ontario-based nonprofit makes radio towers—thin, metal frames that look like old-school television antennae—that are now detecting tagged animals across the globe. Tagged creatures that crawl, step, swim, or fly into the detection radius of a Motus tower—up to 12 miles—will, weather and sightlines permitting, ping the receiver with its unique identifier. The receiver then sends the data to an ever-growing, open-source database of all Motus trackers for other researchers to use: Bat #43190 was here. 

Where the wild bats go

The very same night Hoary #43190 was tagged, Reyes got a ping. The bat had been noticed by a Motus tower in Winters, some 55 miles northeast of Muir Woods. After its mist net experience, it had apparently hightailed it out of the North Bay. That’s when Reyes started checking the Motus logs “obsessively”—as many researchers have come to do.

Like with any good mystery, though, sometimes the answers just bring more questions. Sixteen days later, the bat pinged a tower near Olympia, Wash. “That was like an 800-kilometer trip north during the fall”—when most migratory birds or bats are flying south, toward warmth. “It’s really hard to make sense of the pattern,” says Reyes, who has tagged some 17 other bats since then. Some bats have stayed local, he’s found, while others have moved south. His best guess for the one heading north was that it was looking for someplace nice and chilly to go into torpor—a bat’s lighter version of hibernation.

Now, the tagged bats are “giving us clues on other places to look at” for roosting habitat, Reyes says. He’s hoping, essentially, that they will help him find their friends.

How a hoary bat’s pings show up on the free-to-access Motus dashboard. (Photo courtesy Gabriel Reyes)

Taking the guesswork out of fieldwork

Avian ecologist David Lumpkin remembers all the inconveniences of the world before Motus. Just five years ago, before any towers launched in the Bay, he would go out into the field with a radio transceiver, and its annoying mess of cables, to find birds. “I don’t miss it,” he says. He would try places where he thought his targets might be—known roosts during high tide, or mudflats in low tide—hoping he’d catch a ping.

“Imagine trying to chase the 20, 30 tags you got out over the course of a season,” Lumpkin says. “You could only ever hope to do a fraction of the coverage, with even a few people on the field.” Now the towers are out listening for him—no cables needed.

Dunlin are among the many Bay Area birds being tracked by Motus technology. Others include kestrels, song sparrows, pelicans, and hermit thrushes. (David Lumpkin)

Reyes had fun, too, on those weeks he used to spend hiking through the woods in search of bat roosts. Stalking the wild bats required sheer luck on top of the literal legwork; bat biologists sometimes go years without spotting their research subjects. Now, luck isn’t so essential. And Motus, he says, is a much more efficient way to answer population-level, regional-scale questions. 

Older methods of telemetry, which are still in use in many places, can also get very expensive. For instance, a single GPS tag, which talks to satellites but can only be used on larger animals, may cost $2,000; Motus’s smaller tags run $200 a pop. Sometimes, Reyes’s team would hire a special airplane equipped with telemetry receivers to scan a broad area for leads. 

More towers on the horizon

The denser the tower network is, the easier it is to triangulate an animal’s location. And the network’s going up speedily. Just three years since Lumpkin helped erect the first Motus tower in the Bay Area—at Toms Point Preserve in West Marin—the area now has 18 of them. Researchers from a variety of organizations and agencies including SF Bay Bird Observatory, The Nature Conservancy, the U.S. Geological Survey, and the California Department of Fish and Wildlife have all set up towers. 

“It’s really very scalable,” says Levi Souza, senior environmental scientist at the California Department of Fish and Wildlife, which has set up 13 towers in California thus far, eight of them in the Bay Area. Every additional Motus tower can detect any active tags in the area, so one tower can benefit many research projects.

CDFW gives tags to nonprofits and other agencies—that’s how Reyes got his, for the hoary bats—and the agency is setting up towers on state-owned lands, and has about 40 more in storage waiting to go up. Souza says that some University of California schools have plans to set up towers inside UC reserves as well. “The department is ‘in,’ ” Souza says. “We’re committed to this and we have a lot more to do in this space.”

Conservation is in the details

Lumpkin, whose organization has been involved in long-term shorebird monitoring since 1989, says the “alarming downward trend” of shorebirds prompted Audubon Canyon Ranch to try Motus tagging. They were, previously, tracking just the overall population, monitoring the whole of Tomales Bay. “Now we have much, much more detail on how much individuals move between sites,” Lumpkin says. He and his colleagues can now correlate the birds’ movements with other data, like rainfall, to understand how the birds navigate wet weather or drought. They’re taking blood samples of tagged birds to see how health might factor into their needs, too. 

At the National Parks Science Symposium in 2023, David Lumpkin presented early findings on Motus dunlin tracking, including these pings across California towers. (Photo courtesy of David Lumpkin)

Such analyses could inform policy recommendations that help reverse their decline, he hopes. Lumpkin has a dream, for instance, that the management of the Central Valley’s vast rice fields, an important winter shorebird habitat smack in the middle of the Pacific Flyway, could one day be coordinated with weather events and the movements of dunlins: like a ballet of birds and farmers, choreographed with the help of the Motus network. “It’s a much more accurate picture than we’ve ever had before,” Lumpkin says.


Want to Help Spread the Motus Network?

Private landowners can host towers. Here’s how.

  • A high vantage point is ideal to ensure clear sightlines. Towers can stand on their own or be attached to a property. Maintenance for towers is fairly low—since they are designed for remote places, most are outfitted with their own solar-recharging batteries.
  • Individual towers cost around $10,000 to set up—but federal agencies like the U.S. Fish and Wildlife Service may have towers they need a spot for. And other local nonprofits may work with private landowners, so if you have a potential spot, try calling a local group like the Golden Gate Bird Alliance or the San Francisco Bay Bird Observatory. The Southern Sierra Research Station, a conservation biology nonprofit  based in the Kern River Valley, has helped put up several towers in the Bay Area and beyond, and is looking to add more: call 760-378-3345. 
  • Learn more about what goes into setting up a Motus tower on the Motus station setup website.
  • You can also help advance research using Motus by purchasing tags for species-specific conservation efforts—the Golden Gate Raptor Observatory, for example, is tagging kestrels: learn more on their website
  • At SFFBBO, researchers are keen for tags—two have been deployed on song sparrows, and the organization hopes to tag hermit thrushes and common yellowtails in the future: email outreach director Kristin Butler,

Update, Feb. 26: This story was corrected to reflect that Audubon Canyon Ranch the organization, not David Lumpkin in particular, has been monitoring shorebirds since 1989, and that the birds Lumpkin was monitoring five years ago were not dunlins.

Update, Feb. 22: This story has been updated to reflect that the Motus tags used on the hoary bats are not always solar-powered.

About the Author

Anushuya joined Bay Nature in 2023 as an editorial fellow focusing on Wild Billions, Bay Nature’s project tracking federal money for nature. Before that, she left her hometown of Kathmandu to study journalism at Northwestern University, and has written for InvestigateWest, The Harvey World Herald, and The Daily Northwestern. Outside of the newsroom, you can find her dancing salsa decently well, or playing chess very poorly.

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