Scientists have long known that bad weather can make  disoriented during their annual fall migrations, causing them to wind up in territories they are not accustomed to (a phenomenon called “vagrancy”). However, a team of researchers led by the  (UCLA) has now found that disturbances to the Earth’s magnetic field can also lead birds astray even in perfect weather, and particularly during fall migrations.

With North America’s bird populations steadily declining, discovering the causes of vagrancy could help scientists clarify the threats birds currently face and how they adapt to those threats.

For instance, birds winding up in unfamiliar territory may not find food or suitable habitats, and might die as a result. On the other hand, vagrancy could also turn beneficial sometimes, such as for birds whose traditional habitats become inhospitable due to climate change, and accidentally reach better suited geographic regions.

By comparing data from 2.2 million birds from 152 species that were captured and released between 1960 and 2019 with historic records of geomagnetic disturbances and solar activity, the researchers found that, since birds can sense magnetic fields using magnetoreceptors in their eyes, they are often affected by such disturbances.

“There’s increasing evidence that birds can actually see geomagnetic fields,” said study senior author Morgan Tingley, an associate professor of Ecology and Evolutionary Biology at UCLA. “In familiar areas, birds may navigate by geography, but in some situations it’s easier to use geomagnetism.”

However, when the geomagnetic field is disrupted by, for instance, solar flares or sunspots, birds’ ability to use their magnetoreceptors can be impaired. “If the geomagnetic field experiences disturbance, it’s like using a distorted map that sends the birds off course,” Tingley explained.

The analysis revealed that the closest connection between vagrancy and geomagnetic disturbances occurred during fall migrations, and affected the navigation of both younger and older birds, suggesting that birds rely on geomagnetism regardless of their level of migration experience. 

Moreover, although the scientists expected that geomagnetic disturbances linked to higher solar activity would be associated with the most vagrancy, the evidence showed that solar activity actually reduced the incidence of vagrancy.

One possible explanation of this surprising phenomenon is that radiofrequency activity generated by solar disturbances could make birds’ magnetoreceptors unusable, thus forcing them to navigate by other cues instead. 

“We think the combination of high solar activity and geomagnetic disturbance leads to either a pause in migration or a switch to other cues during fall migration. Interestingly, birds that migrate during the day were generally exceptions to this rule – they were more affected by solar activity,” concluded lead author Benjamin Tonelli, a PhD student at UCLA.