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Birders all over the world are fascinated by vagrants—birds that turn up in unexpected places, well outside their species’ normal ranges. Vagrancy can have many causes, from young birds getting lost due to inexperience to hurricanes blowing migrants off course. But ornithologists from the University of California another reason that birds go astray. Drawing on six decades of bird-banding data, they demonstrated that vagrancy increases during periods of geomagnetic disturbance—a distortion of the Earth's magnetic field that can send birds' internal compasses spinning. 

Benjamin Tonelli, the study's co-author, began his Ph.D. program working under fellow co-author Morgan Tingley in summer 2020, when pandemic lockdowns were still ongoing. His options limited, Tonelli was searching for a research project he could do from home when he stumbled across an article in the New York Times about a correlation between , which are one of several sources of disturbance to the Earth's magnetic field. “And so I thought, I wonder if this is happening for birds,” he says.

By accessing data from the U.S. Geological Survey’s Bird Banding Laboratory—specifically, records of 2 million captures of 152 landbird species spanning 60 years—Tonelli and Morgan were able to look for connections between the rate of vagrancy over time and other factors, including geomagnetic disturbance and solar activity. A clear pattern emerged: Vagrancy during fall migration increased during periods of disruption to the Earth’s magnetic field. For a typical bird species, increased levels of geomagnetic disturbance were associated with a 250 percent jump in the number of banding records from unexpected places or times. The effect was more pronounced for longer-distance migrants.

Researchers have long known that birds rely on the Earth’s magnetic field to navigate during migration, and vagrancy has previously been linked to the same  and solar activity that can cause in the night sky and disrupt the earth's magnetic field. But Tonelli and Morgan are the first to demonstrate on a large scale that geomagnetic disturbances increase vagrancy. However, there was a surprise hiding in the data: High levels of solar activity, which varies on an 11-year cycle and is visible as an increase in the number of black sun spots on the solar surface, appeared to decrease vagrancy.

When analyzing solar activity and geomagnetic disturbance, “I was expecting one or the other or both to be positively associated with vagrancy,” Tonelli says. But the negative relationship between elevated levels of solar activity and vagrancy was completely unexpected. "It turned out there’s a more complex relationship there.”

In addition to the invisible push and pull of the Earth’s magnetic field, migratory birds have myriad other ways to orient themselves during their continent-spanning journeys, including landmarks on the ground and the position of the sun and stars above. Tonelli’s hunch is that, although geomagnetic disturbance can make birds’ internal compasses point in the wrong direction, high enough solar activity can stop those compasses from functioning at all, causing birds to turn to other navigational methods unaffected by space weather.

Migration is such an important event for birds “that there’s selective pressure to have multiple, redundant mechanisms to protect yourself against [times] when, for instance, you can’t use your magnetic compass anymore,” Tonelli says. “You should have some other things that you can fall back on.”

While geomagnetic disturbance is just one of many possible causes of birds turning up in unusual places, by Tonelli and Tingley’s math, it could account for up to 30 percent of vagrancy. Their findings have exciting implications. “I’m sure this will lead to a paradigm shift in vagrancy research,” says Alexander Lees, an ornithologist with England’s Manchester Metropolitan University and author of a book on bird vagrancy, who was not involved in the research. “Imagine -style models that factor into account both terrestrial and extraterrestrial weather to forecast rare bird occurrences!”

Tonelli hopes that birders appreciate the deeper ecological ramifications of the thrilling vagrants that may turn up in their home patches. Vagrancy can have positive effects, such as helping species colonize areas where habitat may be newly suitable due to climate change. However, it can also increase mortality for migrating birds, and perhaps even spread diseases. West Nile virus, for example, may have arrived in North America via a European bird that wandered off-course over the northern Atlantic. “There’s quite a bit of evidence that these rare events in ecology can have really dramatic implications,” Tonelli says.