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New tracking tools reveal bird migration secrets

High-tech bird tracking is taking rapid flight in an era of global scientific cooperation. Birds as small as warblers are signaling their locations from breeding and wintering grounds, from stopovers and long migrations thanks to ever-more-sophisticated miniaturized tracking devices. Scientists harness the power of the sun, satellites, automated radio telemetry, and even the International Space Station to follow birds like never before.

At stake? The future of bird life on Earth. Three billion birds lost since 1970 in North America is incentive alone for scientists coming together and to assure that critical technology is affordable and accessible.

This article was published in the January/February 2021 issue of BirdWatching magazine. Subscribe now!

“We don’t have a lot of time,” says Pete Marra, director of the Georgetown Environment Initiative. “Many species are declining right before our eyes, and we don’t know why.”

Marra is thrilled to see tracking tools applied to smaller and smaller species that help scientists come closer to understanding the full annual and life cycle of birds. While each technology has its pros and cons, together the tools are unraveling mysteries of what birds do when they vanish from our view.

‘Motus can track the smallest animals over the greatest distances and with great precision.’

Kirtland’s Warbler surprise

Wearing tiny radio transmitter backpacks, 100 Kirtland’s Warblers guided Smithsonian and Georgetown University scientists to a conservation revelation in 2020. Far from staying put after a 1,700-mile trek from the Bahamas to northern Michigan’s scrubby Jack pine forests, some of these rare warblers kept winging long distances and often at night.

Applying the Motus Wildlife Tracking System, an international collaborative research network that uses automated radio telemetry, biologists analyzed surprising location data from birds bearing uniquely coded nanotags.

Lead researcher Nathan Cooper of the Smithsonian Migratory Bird Center found that 11 percent of the breeding birds and 60 percent of non-breeders explored much more than the few miles he expected they’d fly within their specialized habitats.

“I was pretty shocked,” Cooper says. “I’ve been interested in these floaters, or non-breeding birds, for a long time.” He had no idea that a single bird would cover almost 50 miles in a go and 300 miles over the summer season.

The implication? Cooper anticipates scientists finding similar long-distance movements in other birds. Follow the birds, and they will tell us the ranges they need — and those are likely larger in size than previously understood.

“For songbirds, this is one of our first opportunities to look for behavior like this because of prior technological limitations,” he says.

Another twist of the research: The birds divulged answers to questions that Cooper hadn’t asked in his original quest to study how tropical wintering conditions affect migrating and breeding success. That’s why his team tagged the birds in the Bahamas. This fortuitous choice assured they would capture some of the floaters that would otherwise have been tricky to lure into mist nets during mating season.

“It was just lucky,” Cooper says. “I made an animation of the data and noticed all this movement happening during the breeding season and zoomed in and made this discovery.”

Very little is known about birds that aren’t raising chicks. Are they prospecting for future nest sites? With the latest technology, birds are signaling new trajectories of study and reminding scientists to be curious and receptive to surprise findings.
A Motus tower stands in a field in Oscoda, Michigan. More than 900 such stations track birds from at least 225 species on four continents around the globe. Photo by Nathan W. Cooper

900 stations and counting

Stuart Mackenzie manages the Motus Wildlife Tracking System and directs Migration Ecology for Birds Canada, headquartered in Ontario. He stresses the significance of the community science made possible through a coordinated and collaborative approach to migratory animal research. The cornerstone is the centralized database and research software platform housed at Birds Canada’s National Data Centre.

“Motus is contagious — a couple projects, or a couple of receiving stations, get underway, and then it snowballs,” Mackenzie says. From modest beginnings in 2013 in the Canadian Maritimes and New England states, the network now encompasses over 900 stations and 330-plus projects in 31 countries on four continents. Motus has tracked over 24,500 individuals of more than 225 species.

Each registered tag transmits on the same frequency. The tag’s unique code and fixed rate, a burst interval, sends a signal that identifies an individual. Anyone can put up a receiving station that is, in turn, available to others with registered tags. Nominal fees pay for managing the centralized data. A station costs from $5,000 to $10,000, depending on the setup. After tag detection files upload to the central database, researchers receive a summary usually within minutes.

“We are seeing some groundbreaking and remarkable studies come out of Motus,” says Mackenzie. “Motus can track the smallest animals over the greatest distances and with great precision.”

Radio transmitters weigh as little as 0.2 grams (a small paperclip is 1 gram), light enough to trace even monarch butterflies and darner dragonflies. The batteries on the tiniest tags last up to one month and the largest up to three years.
IMPACTS: Research using Motus stations uncovered new information about the migratory habits of Swainson’s Thrush and its journeys from South America to Canada. Photo by Ana M. GonzálezIMPACTS: Research using Motus stations uncovered new information about the migratory habits of Swainson’s Thrush and its journeys from South America to Canada. Photo by Ana M. González

New discoveries

“The early bird gets the worm” cliché seemed to apply to Swainson’s Thrushes heading north sooner in spring from native forests and shade-grown coffee plantations of the eastern Andean Mountains in Colombia. The accepted general premise is that neotropical migratory songbirds leaving earlier will get a head start on mating, nesting, and raising chicks in North America.

In 2020, scientist Ana M. González published her discovery of a different migration tactic, a finding made possible through Motus tracking. Swainson’s Thrushes could either depart early and fly slowly to their destinations in the northern states and Canada or leave late and fly faster to arrive almost at the same time.

She determined that thrushes in the native forests stayed longer, likely to keep fueling up on fruits, berries, and insects. With excellent nutrition, they had the power to catch up. Spending less time on the journey means less exposure to storms and other dangers.

Swainson’s Thrushes join other migratory birds in peril that winter only in the high-elevation montane forests of the Andes. Logging forces the birds into small patches of remaining native forests. Shade-grown coffee plantations that protect a high diversity of native trees serve as critical refuges, too.

González has studied Swainson’s Thrushes in her native Colombia since 2015, the year she placed a bird band and then a harness with radio-transmitter on “Pecas” (freckles in Spanish), a young bird born in 2014 spending his first winter in South America. The moment she set him free to fly on March 19, 2015, marked the beginning of a stunning expedition. Nearby Motus stations tracked Pecas’ local movements and the momentous night of April 12, when he flew north. A month later, the intrepid traveler flew past a small array of Motus stations at Chaplin Lake, Saskatchewan — a migration of more than 3,700 miles in 34 days and covering at least 109 miles per day.

For her Ph.D. dissertation, González radio-tagged 268 Swainson’s Thrushes over three years; 44 were detected on their northward migration. She found that adult birds in forests departed up to 10 days later than birds in shade-grown coffee plantations. That’s when her studies revealed later birds migrating faster.

The Motus telemetry array also filled in gaps of spring migration routes — noting birds along the border between Colombia and Panama and on the Gulf Coast. The information suggests an aerial pathway through Central America to a trans-Gulf crossing, perhaps from the Yucatan Peninsula. Far to the north, Motus picked up thrush signals in both Quebec and Saskatchewan, implying an eastern and western route through the U.S.

Among thrushes tagged in South America, Motus located about 40 percent in North America. For other species, it’s as high as 60 to 70 percent. Consider how that compares to bird banding. Since 1963, more than 3,100 Swainson’s and Gray-cheeked Thrushes were banded on their Colombia wintering grounds. Only six have been encountered again.

In the Journal of Animal Ecology, González also reported on the role of winter habitat quality on migratory behaviors of Swainson’s Thrushes. She and her team took small blood samples from thrushes caught in mist nets to measure stable carbon isotopes and associated moisture patterns.

Since 1999, scientists have applied the tool of analyzing stable isotopes to sleuth animal movements through habitats. Higher moisture in the thrush analysis suggests favorable pre-migratory conditions. Birds departed later from moister, high-quality forests. And they all left early from both native and shade-grown coffee forests during the driest year.

González’s work tells the bigger story of complex linkages in a migratory bird’s life cycle, especially in this era of climate change. Advanced tracking comes ever closer to spotting what habitat is intact, what’s broken, and what might yet be repaired.
Motus towers track birds locations when an animal wearing a small radio nanotag flies past a tower. Tim Romano/Smithsonian Conservation Biology InstituteMotus towers track birds locations when an animal wearing a small radio nanotag flies past a tower. Tim Romano/Smithsonian Conservation Biology Institute

Motus heads west

A map of North America’s Motus stations shows thick clusters of receivers east of the Continental Divide and particularly in the Northeast, where the project started. The strategy of linking necklaces of stations across North-South flyways assures picking up the most birds.

In Oregon, where I live, the Klamath Bird Observatory took the lead in 2020 with the first two Motus receivers in the state. A partnership with the MPG Ranch in Montana (profiled in the May/June 2020 issue of BirdWatching) helped erect the stations in southwest Oregon.

“With Motus technology, we are ramping it up in the West, and there is a lot of momentum,” says Jaime Stephens, the observatory’s science director.

Tagged Lewis’s Woodpeckers, for example, are transmitting signals within their wintering area in Oregon. The species has declined approximately 70 percent in the past half-century. Scientists have identified threats that include logging of older decaying trees and standing snags. To safeguard the bird’s future, they need an accurate roadmap; data from Motus should come in handy.

The Klamath Bird Observatory, along with American Bird Conservancy and the Center for Natural Lands Management, are also studying Oregon Vesper Sparrows. A new Motus station in the aptly named Vesper Meadows will focus on the survival of chicks in their first few months before they migrate in mid to late September. This subspecies has vanished from British Columbia and northwestern California, and it has been proposed for listing under the Endangered Species Act. Scientists estimate fewer than 3,000 birds remain.

“We are applying multiple technologies to answer questions about the Vesper Sparrow,” Stephens says. As Motus picks up the daily movements of fledglings in grassland montane meadows, the young birds will likely unveil behavior never witnessed before. Other sparrows carry miniaturized GPS tags to identify precise locations within wintering homes that may be in California’s Central Valley. To find out, the researchers must recapture the birds the next year in Oregon and read the data.

John Alexander, director and founder of Klamath Bird Observatory, says unprecedented and international partnerships are essential to recover western birds. “There’s an incredible urgency, and the first thing to learn is where the birds are going, where they are limited, and to focus conservation in the right places,” he says.
IMPERILED: An Oregon Vesper Sparrow carries a GPS tag. The subspecies has never been tracked year-round before, and new research aims to uncover important information about its migratory routes and overwintering areas. Photo by Frank LospallutoIMPERILED: An Oregon Vesper Sparrow carries a GPS tag. The subspecies has never been tracked year-round before, and new research aims to uncover important information about its migratory routes and overwintering areas. Photo by Frank Lospalluto

An all-seeing eye

The newest entry into the high-tech scene is ICARUS (International Cooperation for Animal Research Using Space), a project of the Max Planck Society, Germany’s premier independent research organization. Transmitters on tagged wildlife beam information from the International Space Station that orbits the Earth 16 times a day and covers about 80 percent of our planet’s surface between the 55th parallel north and 55th parallel south. The tags relay details on a bird or animal’s location, physiology, and environment.

After two years of testing, the first global research project launched in September 2020 to follow several thousand blackbirds and thrushes in Europe, Russia, and North America.

Roland Kays, professor at North Carolina State University, is on the scientific advisory board for ICARUS and helps distribute the North American tags. The key innovation, he explains, is in crafting lighter-weight tags than traditional satellite tags that are too heavy for small birds to carry. The Space Station circuits about 200 miles above the Earth compared to the global Argos satellite at 520 miles. Closer proximity means the tags do not need as strong a signal and therefore can be lighter. The solar-powered tags transmit only when the satellite is overhead and cost about $500 apiece.

“We are starting with robins, in part to compare with a study on Eurasian Blackbirds, and will be expanding to a variety of other species we know little about their migration, especially some declining species,” Kays says.
Elly Knight of the University of Alberta (top) holds a Common Nighthawk for a migration research project. Photo by Jonathan DeMoorElly Knight of the University of Alberta (top) holds a Common Nighthawk for a migration research project. Photo by Jonathan DeMoor

Shedding light on nighthawks

Researcher Elly Knight monitors the crepuscular flights of Common Nighthawks in Canada by recording their peeeent calls and the wing-boom of diving males in courtship. In this emerging field of bioacoustics, she’s finding their key breeding areas. Like the popular trail cameras that take video and photos of wildlife passing by, the mounted boxes record nature’s symphony. Knight applies computer processing to identify Common Nighthawks within the recordings.

She also traces their migrations to South America and back, analyzing near-real-time data from the newest and lightest satellite tags. One bird flew 2,200 miles in a nonstop flight over five days from Colombia to central Texas, averaging 440 miles a day.

“They are strong enough fliers to take the Gulf of Mexico in a single shot,” Knight says, noting that their flights are nocturnal except when it comes to the long stretches.

The data she collects is part of the Common Nighthawk Migratory Connectivity Project, a collaboration of Smithsonian Migratory Bird Center, Environment and Climate Change Canada, and the University of Alberta, where Knight is completing her Ph.D. The team tagged 63 birds in 12 different locations, including Arizona, where the birds are steeply declining, and Oregon, where they seem to be faring better. The far-ranging nighthawk has fallen in numbers by
61 percent from 1966 to 2014, according to Breeding Bird Surveys.
A nighthawk wearing a transmitter awaits release. Photo by Jonathan DeMoorA nighthawk wearing a transmitter awaits release. Photo by Jonathan DeMoor

Satellite tags relay locations

The advantage of satellite tags is the ability to relay immediate information from far-flung parts of the world, compared to Motus that requires telemetry towers. The tradeoff is the battery power required and whether the satellite picks up the transmission. Satellite tags tend to be pricey but are becoming more affordable. A GPS-Argos tag is about $1,500. While still too heavy for birds smaller than nighthawks, they have lightened considerably with technology advances in the past few years.

Bird researchers apply two types of satellite tags — GPS and Argos. GPS tags record the time and position from a satellite to calculate an exact location, a technology first used by the military in the 1980s and made available to the public in 2000. The 31 GPS satellites are what many of us rely on for navigating via smartphone.

The Argos tags relay locations up to a satellite that are then sent back to receivers on Earth. Launched in 1978 by NOAA and the French Space Agency, the now seven orbiting Argos satellites are dedicated to tracking animals with a goal of better protecting our environment.

The nighthawks flew with harnesses holding what’s called “pinpoint GPS Argos tags,” the smallest and lightest satellite tags available at .1 ounce (3.5 grams). Any heavier would be too much for nighthawks that weigh only about 3 ounces (85 grams). In comparison, Kirtland’s Warblers weigh about half of an ounce (15 grams).

Knight and the other scientists in the study programmed the tags to receive and upload information at certain times and dates. As of a 2018 publication, their tracking has confirmed that the birds return to within a half-mile (1 km) of nests, and their winter home falls within Brazil’s Amazon and Cerrado regions, the latter a mosaic of savannah, grassland, and forest that’s under siege from land clearing. Some but not all tagged nighthawks winter in contiguous rainforest.

“They are nocturnal and highly cryptic and hard to find on the wintering grounds,” says Knight. That partly explains why so little is known about their behavior in the Cerrado, which is also a lightly birded area.

The researchers also placed GPS archival tags on three nighthawks in northern Alberta. Knight retrieved one. The tags store satellite data on microcomputers and are more affordable, but the birds must be recaptured after their long flights from Brazil.

“Tagged nighthawks are insanely hard to re-catch,” says Knight, noting their home range is 25 square miles. The problem with tagging initially is not knowing whether a bird caught in a mist net was out foraging or close to a nest camouflaged on the ground.

That one bird provided 700 GPS location points on its 2,200-mile nonstop flight. Might this be common or was this one superlative flyer?

“Nighthawks are a species we know so little about,” says Knight. “We’ve started from scratch, and that’s given us the ability to work collaboratively and standardize our research.”

The scientists are cross-checking with Motus to pin down departure and arrival dates in Ontario. The goal: to identify the “pinch points” for nighthawks that could lead to targeted conservation for the aerial acrobats.

‘There’s an incredible urgency, and the first thing to learn is where the birds are going, where they are limited, and to focus conservation in the right places.’

Ambassadors for conservation

Across the world, birds bearing minipacks with antennae are serving as nature’s reporters and ambassadors. A Yellow-billed Cuckoo equipped with a satellite transmitter lofts from a favorite tree perch in Chattanooga, Tennessee, and flaps south to the Gran Chaco of Paraguay and Bolivia. Its fall migration is no longer existential but vividly portrayed for scientists who can identify risks along the way.

Once the birds touch down in tropical forests that have sheltered generations of cuckoos, they face accelerating deforestation, as farmers cut down vast swathes of trees to plant soy.

Why care about the fate of forests far from the nesting homes of cuckoos? The tracked birds expand our definition of home to caring for the fate of Gran Chaco, too.

“The cuckoos are like wizards dancing across the treetops that we hear more than we see them,” says Georgetown Environment Initiative’s Marra, who is supervising a study of the species.

To lose the magic of cuckoos would be one more unraveling of diversity and natural wonder. Marra has long championed the science of migratory connectivity and the full annual cycle of birds. He’s witnessed, too, the power of communicating science in the Washington, D.C., area, where he lives.

“When I tell people that the Gray Catbirds here spend most of the year in Cuba, they’re excited,” he says. “We share our birds. In fact, the migratory birds that fly through the nation’s capital city represent 27 to 30 countries.”

Every bird is a messenger. In this bright age of tracking, scientists have taken a cue from the birds that freely cross political borders. Their collective, international work is vital for conservation and for telling the spellbinding stories of birds on the move.

To catch a warbler

READY TO FLY: A Hermit Warbler wearing a geolocator on its back and leg bands rests in a researcher’s grip. Photo by Hankyu KimREADY TO FLY: A Hermit Warbler wearing a geolocator on its back and leg bands rests in a researcher’s grip. Photo by Hankyu Kim

Before a warbler takes to the sky with a tiny geolocator, researchers must catch it, and that is not always easy. Just ask Hankyu Kim, who is completing his Ph.D. dissertation at Oregon State University examining the routes, behaviors, and habitats of Hermit Warblers. The elusive warblers nest only in Washington, Oregon, and California within mature coniferous forests and flit among the towering tree canopies. They migrate about 2,000 miles to Mexico and Central America.

Using playback calls and a plastic decoy Hermit Warbler, Kim lured male warblers down from above to fly into mist nets set up in the H.J. Andrews Experimental Forest in Oregon.

“One bird took six hours to catch,” he says. “We tried many different net formations.”

Birds often become wary of mist nets and playback calls, making capturing them difficult. That makes the feat of equipping more than 150 Hermit Warblers with geolocators over two seasons even more impressive. Kim and his team recovered 25 tagged birds that returned close to where they were captured. Geolocators periodically record ambient light level to determine location. Preliminary results from retrieving the data show that individual birds scattered in Mexico — wintering hundreds of miles apart within high montane forests.

Kim and partnering organizations are still piecing together the life cycle of a declining songbird that favors higher elevation evergreen forests of multiple species, with trees of immense girth and height. In Oregon, he seeks to understand how these now rare ancient forests’ cooler temperatures and layered structure may be key to survival in a warming world.

Maps from eBird show where to look for interesting birds at different times of year.

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