How Scientists Learned About The Blackpoll Warbler’s Epic Migration

We have entered a golden age of animal tracking. The same technological boom that put smart phones and tablets in our hands now allows us to place miniature sensors on animals, which has revolutionized the way that we study bird migration. Whereas the paths that individual species take during migration was once a mystery, we are now learning where birds are moving throughout the year, illuminating the various threats they face as they migrate across entire continents and oceans.  

Light-level geolocators are one of the cutting-edge technologies that have been a major component of this revolution in tracking bird migration. Ironically, the way they determine positions is based on celestial navigation that was used by the earliest explorers. But, rather than determining global coordinates at a desk with maps piled high and intricate navigation tools, geolocator sensors are determined by a computer. 

“It’s the same concept that was used for centuries for people to navigate around the globe,” says Bill DeLuca, a migration ecologist for Audubon’s Migratory Bird Initiative. “Day length varies with latitude, based on date. So if you know the date and you know how long the day is, you can get some estimate of latitude. If you know sunrise and sunset, you can also calculate when the sun was directly overhead, and use that to estimate longitude. By sensing the light level for almost every minute of the day, for every day of the year, you can see how these geolocators can give you general location estimates as a bird migrates throughout the hemisphere.”  

DeLuca has been studying the drivers of bird distribution and population changes to inform conservation for more than a decade. He says that, for the longest time, not knowing exactly where birds overwintered represented a massive research gap that limited conservation efforts. 

“How do you conserve something if you don’t know where it is for nine to ten months out of the year, let alone knowing when might be the most important time for maintaining a species’ population?” DeLuca asks. Having that piece of information is “such a big deal” for bird conservation, he added. 

Geolocators have played an integral role in helping us obtain that information. Researchers attach light sensors to birds like a backpack, which the birds then carry around, recording light levels as they move throughout the year. When birds are recaptured, researchers can take the backpack sensor and run it through a series of analyses that determine the latitudes and longitudes of that bird’s travels. 

While geolocators aren’t the only technology capable of doing this, part of their value is in their miniature size. 

“The general rule is that the backpack can’t be more than five percent of the bird’s total weight. So for 12-gram birds, it has to be less than 0.6 grams,” says DeLuca.  

For a long time, this rule precluded researchers from tracking small songbirds, such as warblers. But because geolocator sensors have gotten increasingly lighter, they have allowed us to study the annual cycle of species that were previously untrackable. In particular, this has allowed us to start tracking songbirds like thrushes and warblers, which were previously off limits. 

DeLuca had been studying Blackpoll Warblers for years, and had questions about their overwintering ecology. When he and his colleagues realized that geolocator sensors were light enough to track the songbird, they jumped at the opportunity to learn where this bird spent a majority of the year. 

“It was the first research project of its kind for this species—it was the first year that geolocators had become small enough,” says DeLuca.

His study revealed a truly epic migration story: Blackpoll Warblers breed in the northeastern United States and travel nonstop more than 2000 kilometers across the Atlantic Ocean to winter in Hispaniola and Puerto Rico—quite a feat for a bird weighing as much as a AAA battery.  

This study is representative of the many migration stories that these little backpacks have offered conservation efforts since it was published in 2015. The field has only pushed forward. “Since then we’ve been able to track even smaller birds,” says DeLuca. “For example, Prairie Warblers, which are really small, weighing only around seven to eight grams can now be tracked with geolocators.”

DeLuca has continued to use geolocators to uncover migration pathways. His most recent study, published in 2019, followed Blackpoll Warblers all the way from Nome, Alaska, across North America, then over the Atlantic on their way to the Amazon and back.  

With these developing tools, researchers are able to track more and more species and, in wake of this golden age of migration tracking, Audubon’s Migratory Bird Initiative is hoping to synthesize their findings to visualize the spectacle of migration and inform conservation action. Given that DeLuca helped push the field forward in the past, joining Audubon’s Migratory Bird Initiative is fitting for what he sees as the next steps.  

“It’s cool to see where the field is going,” DeLuca says. “We’re past the point of just describing where birds are moving. Now that we have a better picture of where birds spend their time throughout the year, we can start to figure which parts of the year are most important for their conservation, which is very exciting.”  

The Migratory Bird Initiative will bring together the latest spatial information on species distributions and movements across their annual cycles to identify priority areas for 520 species of migratory birds. We will use this information to define where and how to focus Audubon’s conservation investments in order to protect, restore and manage key habitat and also mitigate threats along full migratory pathways for these species.  

If you are a researcher and would like to contribute data to Audubon's Migratory Bird Initiative, please fill out this Data Sharing Agreement. If you would like to contact us for more information, please email migratorybirds@audubon.org.