An example of an extreme vagrant, this Icterine Warbler – a species that normally nests in northern Europe with a winter range in southern Africa – was photographed last September on St. Lawrence Island in the Bering Sea of northwest Alaska by Rodney Ungwiluck Jr. The documentation of this off-course bird created a First North American Record and a First State Record. Perhaps this Icterine Warbler was drawn so far beyond its normal range by changes in the Earth’s magnetic field.

We are well aware of the excitement created when birders find off-course birds – vagrants – after all we dedicate a weekly Rare Birds article to report remarkable birds found far from their species’ normal range. Now a new study by UCLA ecologists explores one reason some birds are drawn off-course: Disturbances to Earth’s magnetic field can lead birds off-course, even during perfect weather, especially during fall migration. And to make their research more meaningful to birders, the team developed an internet-based tool that tracks geomagnetic conditions and predicts the potential of vagrancy in real time.

Published in Scientific Reports, this new study assesses how the causes of vagrancy can help biologists better understand threats that birds encounter beyond their usual ranges and the ways they adapt to those threats. For example, birds that wind up in unfamiliar territory are likely to face challenges finding food and habitats that suit them, and may die as a result. But there could be some benefits for species with traditional ranges that are becoming more uninhabitable due to climate change when individuals are “accidentally” introduced into geographic regions that are better suited for them.

Earth’s magnetic field, which runs between the North and South Poles, is generated by several factors, both above and below the planet’s surface. Decades of lab research suggests that birds can sense magnetic fields using magnetoreceptors in their eyes. The new UCLA study lends support to that research from an ecological perspective.

“There is increasing evidence that birds can actually see geomagnetic fields,” said Morgan Tingley, the paper’s corresponding author and a UCLA associate professor of ecology and evolutionary biology. “In familiar areas, birds may navigate by geography, but in some situations it’s easier to use geomagnetism.”

But birds’ ability to navigate using geomagnetic fields can be impaired when magnetic fields are disturbed, by the sun’s magnetic field for example, particularly during periods of increased solar activity, such as when sunspots or solar flares act up, but also from other sources. “If the geomagnetic field experiences disturbance, it’s like using a distorted map that sends the birds off-course,” Tingley explained.

Lead UCLA researcher Benjamin Tonelli, worked with Tingley and Casey Youngflesh to compare data from 2.2 million birds representing 152 species that were captured, banded, and released between 1960 and 2019 – part of a US Geological Survey tracking program – against historic records of geomagnetic disturbances and solar activity.

While other factors such as weather probably play bigger roles in causing bird vagrancy, the researchers found a strong correlation between birds that were captured far outside of their expected range and the geomagnetic disturbances that occurred during both fall and spring migrations. But the researchers noted the relationship was particularly pronounced during fall migration.

Geomagnetic disturbances affected the navigation of both young birds and seasoned adults, suggesting that birds rely similarly on geomagnetism, regardless of their level of migration experience.

The researchers expected that geomagnetic disturbances associated with heightened solar activity would be associated with the most vagrancies. To their surprise, solar activity actually reduced the incidence of vagrancy. One possible reason is that radiofrequency activity generated by the solar disturbances could negate birds’ magnetoreceptors, leaving birds 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,” Tonelli explained. “Interestingly, birds that migrate during the day were generally exceptions to this rule – they were more affected by solar activity.”

Although the researchers only studied birds, their methods and findings could help scientists understand why other migratory species, including whales, become disoriented or stranded far from their usual territory. “This research was actually inspired by whale strandings, and we hope our work will help other scientists who study animal navigation,” Tingley noted.

To make the research more accessible to birders, the team has developed a web-based tool that tracks geomagnetic conditions and predicts vagrancy in real time. The tracker is offline during the winter, but it will go live again this spring, when migration begins.

To refer to the original article published in ScienceDaily, see When migrating birds go astray, disturbances in magnetic field may be partly to blame: Study could help scientists better understand threats to birds -- and their ability to adapt -- ScienceDaily