44-hour Flights: Tracking Curlews Across North America

How do you study a species that travels thousands of miles, using a mode of transportation that is difficult or impossible for humans? This is the tricky reality of studying migratory birds whose incredible journeys happen far from the ground. Fortunately, modern technology is helping us learn more about the phenomenon of their migration.

The same weather radars we use to make weekly forecasts and track storms can also detect flocks of flying birds. We can learn about the movements of whole populations across continents and even start to forecast bird migrations. GPS tracking devices give us a more detailed look at individual birds, wherever they travel (even across oceans). With these tools, we can learn how many birds migrate and when, the routes they take, and where they stop along the way.

We can even learn how a bird’s speed, altitude and body orientation change in flight. We call these “in-flight behaviors.” We are already finding that many birds fly farther, faster and higher than we thought. And when we consider that in-flight behaviors show us the relationship between birds and their aerial habitats, we can start to unravel the mysteries of how birds achieve their amazing flights.  

In 2019, Smithsonian researchers began a project with the Intermountain Bird Observatory at Boise State University, American Prairie and the University of Oklahoma’s Aeroecology Group to study and conserve long-billed curlews. Curlews are an iconic bird species of the Central Great Plains. The population we study nests in North-central Montana but migrates through the Great Plains and winters south of the U.S./Mexico border in coastal lagoons and the high desert.

GPS tags are our window into the lives of these birds, helping us understand their needs and the challenges they face throughout every stage of their lifecycle. Placing a GPS tag on a wild bird gives us access to incredible information that can help conserve entire populations. We use tiny tags, and our protocols are designed to maximize safety and minimize stress for the birds. We understand that tagging may still cause some small disturbance for these endurance athletes, so we try to squeeze every last drop of information out of the tags to achieve the maximum conservation benefit for curlews everywhere.

At the start of the project, we equipped seven long-billed curlews with tracking tags. The tags collect GPS points with the kind of high accuracy your phone has if you use it to navigate while driving. We tagged 20 more birds the following year and another eight in 2021. These tags are some of the most advanced, lightweight tags available. They transmit over the cell phone network, so we can access data in real-time.

In addition to a bird’s latitude, longitude, and altitude, the tags collect in-flight behavior data. They are equipped with triaxial accelerometers, tools that measure acceleration and help us identify types of movement — like flying, foraging or resting. We use all this data to create a 3-D point of where a curlew was in the air. When paired with atmospheric data — like temperature, wind and pressure — we get a picture of how curlews respond to changing conditions as they fly. For example, a curlew may change its flying altitude or adjust its heading.

Next, we can begin to ask (and answer) important scientific questions, like what weather conditions might explain in-flight behaviors like changes in flying altitude? And how much do birds rely on favorable winds during migration? (Think about how moving in the same direction as the wind makes you go faster — the same is true for birds!). With climate change, these questions have new urgency.

Climate change is already causing spring to arrive earlier. It’s also expected to alter atmospheric conditions, like the strength of winds. So, what might that mean for birds? As birds travel earlier or faster to keep up with the advancing spring, will they have the same wind support they evolved with to complete their journeys? If not, will they use more energy? And how might that affect them once they reach their destination?

We can also think about ways to conserve aerial habitat for migratory birds, just like we protect their habitat on land. Where should we create safe sky spaces, free of drones and planes? How can we prevent light, chemical and sound pollution in areas where birds travel? Where should we avoid building wind turbines and radio towers?

Answering these big questions will take more time and data, but the long-billed curlew’s fall 2020 migration is already brimming with interesting results. The curlews typically started long flights in the evenings and often continued flying into the daylight hours. Their entire trips totaled around 45 hours, with each long leg lasting 10 to 25 hours. One female curlew, called Tapes, flew the entire route from Montana to Central Mexico in one 44-hour flight, covering 1,641 miles (2,641 kilometers).

During the first part of an evening flight at the start of migration, a female curlew called Carmen flew at high altitudes. She returned to high altitudes again during the daylight hours, as she continued her journey south. Carmen also had the quickest ground speeds when she flew at higher altitudes, suggesting she may have found favorable tailwinds there.

It’s exciting to analyze these kinds of data. They reveal details about the lives of long-billed curlews that would otherwise remain secret as the birds travel to and from their far-apart destinations. We still have much to learn. But with their high-tech tracking tags, there is no doubt the curlews will be teaching us more about how they use their important aerial habitats.