Where Do Songbirds Go to Die?
After revolutionizing bird-migration science over the past decade, geolocation technology is poised to shine light on the field’s darkest mystery.
One afternoon in May 2008, a graduate student named Pat Kramer was in northwestern Pennsylvania catching purple martins. The bird, a large swallow that nests in artificial birdhouses across North America, is a well-studied species. But one particular purple martin Kramer and some fellow researchers from York University caught was about to revolutionize ornithology.
Kramer let an exclamation mark creep into his otherwise staid field notebook when he found it: “At 2:45p.m., Evelina captured Yellow 2551 in WH-43!”
Yellow 2551, the identification code assigned to this martin, was wearing a geolocator, a small device that uses a light sensor to calculate latitude and longitude and track a bird’s movement over time. The geolocator had traced this female martin’s migratory journey to Brazil and back via the Yucatan Peninsula. In doing so, it provided the first data on what had been a massive blind spot in the scientific understanding of the otherwise familiar purple martin: Where, specifically, does the bird go during migration? And what route does it take?
In the decade since Yellow 2551’s pioneering journey, scientists have used geolocators and a variety of newer technologies to gain an increasingly sophisticated understanding of how migratory songbirds move across the globe. As a result, a much more nuanced picture has emerged of how conditions on wintering grounds and along migration corridors affect birds’ survival. And very soon, with the deployment of some cutting-edge gadgetry on the International Space Station, ornithologists will finally be able to delve into the most disturbing mystery of all: why half of the migratory songbird species in North America are disappearing at alarming rates.
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As so often happens, all this began with a coincidence. At a conference in Mexico, Bridget Stutchbury, an ornithologist at York University, stumbled across a poster on geolocators, which the British Antarctic Survey first developed to study seabirds. She realized that at 1.5 grams, the geolocators were small enough to put on very large songbirds like the purple martin. The following year, Stutchbury and her students deployed 20 of them, strapped to the birds with a backpack-style harness. Though they recovered just two from this initial batch—geolocators are “archival,” meaning they don’t transmit data remotely—they revealed an immediate surprise: Yellow 2551 had flown north in the spring much quicker than expected, covering the thousands of miles from Brazil to Pennsylvania in just two weeks.
“It was the first time anyone had been able to track songbirds from start to finish in their annual migration,” recalls Stutchbury. “We know conceptually that they do it, but to see it on the map ... it’s like, ‘Yes, the bird did this. Here’s the proof.’”
The research made a splash after it came out in Science in 2009, and soon the journals were regularly publishing geolocator studies of migrating songbirds. Through September 2017, a total of 121 such papers have been published, according to a forthcoming review study by Emily McKinnon, a postdoctoral fellow at the University of Windsor. Many of them tell remarkable stories. After recovering geolocators from two Connecticut warblers in Manitoba in 2016, for instance, McKinnon was astonished to discover that the tiny birds’ journey south to Bolivia the previous fall had included a nonstop, two-day haul over the Atlantic Ocean.
“One of the things that really continues to be amazing is that these small songbirds are doing things that we did not think that they are capable of,” she says.
This geolocator science is filling in fundamental gaps in the natural histories of entire species. Stutchbury eventually tracked about 400 purple martins with geolocators, and showed that the species’ core wintering range is in the Brazilian Amazon, rather than more densely populated areas of the country much further south and east, as scientists had previously thought.
In 2017, understanding of purple-martin migration took another leap forward thanks to archival GPS tags, which log birds’ locations down to just 10 meters, as opposed to geolocators’ several-hundred-kilometer margin of error. The research, led by Kevin Fraser at the University of Manitoba, showed that purple martins spend significant amounts of time on low-lying river sandbars in the Amazon. At this level of precision, specific conservation implications begin to emerge. Protecting this sandbar habitat, for example, could be key to maximizing purple martins’ winter survival.
Another technology out there is called Motus, which uses radio telemetry and is non-archival: Special towers simply record the presence of Motus-tagged birds that pass within range (though that range is small and there aren’t many towers out there). There are now satellite tracking tags small enough to use on large songbirds, although weight still prevents them from being widely applied in songbird research. And laboratory techniques like stable-isotope analysis of bird feathers also make valuable contributions by showing, for example, where birds pause during the year to molt new feathers.
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Together, it all furthers the increasingly nuanced understanding of songbirds’ full annual cycles from breeding grounds to winter hideouts and back again, and the intricate cause-and-effect relationships between them that are only just now becoming clear. “We’re basically rewriting the textbooks,” says Pete Marra, head of the Smithsonian Institute’s Migratory Bird Center.
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One final frontier remains, however: Where and how do birds die? Recovering data from archival tracking devices depends on birds surviving migration—and researchers catching them again afterward. That means that the many, many migratory songbirds that don’t survive their journeys simply disappear into oblivion, telling no tales about the circumstances of their demise. “If we can’t figure out where they die, then we can’t figure out why they’re dying, and we can’t then implement conservation strategies to stop those declines,” says Marra.
That’s what could make the ICARUS initiative—short for “International Cooperation for Animal Research Using Space”—the next ornithological game changer. Scheduled to come online next year, after Russian cosmonauts install a new antenna on the International Space Station, ICARUS will remotely track tagged birds’ movements with such precision that it will be able to tell when they stop breathing, says Martin Wikelski, the project leader at the Max Planck Institute for Ornithology.
When ICARUS launches, the lightest tags will weigh about 3.5 grams—still too heavy for most songbirds—and they’ll only communicate with the orbiting antenna once a day. But Wikelski predicts a one-gram tag will be available in two to three years, and plans are in the works to put more antennae in orbit, improving ICARUS’s coverage and allowing more frequent data transmissions. The idea is to allow scientists to follow individual birds across the globe while keeping tabs on an enormous amount of data—speed, altitude, temperature, heading, acceleration, and so forth—much like airplane passengers now track their flights on the seat-back display. As tags get smaller and satellite coverage improves, the birds they track can be smaller and the data they receive gets closer and closer to real time.
This means that any interested party should soon be able to follow a dot on a computer screen representing an individual purple martin en route from Pennsylvania to some tiny speck of a sandbar in the Amazon. It is, obviously, an exciting prospect for the ornithologists who study this stuff, as well as for the wider bird-watching public. Wikelski emphasizes the educational angle: Imagine an elementary-school science unit built around a single purple martin’s journey to Brazil and back again.
Perhaps that purple martin makes that journey just fine. But then again, maybe the bird fatigues and plunges, Icarus-like, into the sea. Or maybe it goes winging gracefully into an office tower’s plate-glass windows. Maybe some other fate befalls it. That might upset those elementary students, but, with ICARUS beaming back all the gruesome details, this dead bird would no longer be another scientifically useless tick mark in the mortality column. Instead, it would do its part in revolutionizing scientists’ understanding of songbird migration yet again, by showing exactly what happens to the many migratory songbirds who don’t make it back home—and by pointing to ways to keep them migrating in the future.