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It seems quite logical that bad weather can sometimes disorient birds on their annual fall migrations, causing them to end up in territory they are not used to. But why, even when weather is not a major factor, do birds stray from their usual routes?
A new paper by UCLA ecologists explores one reason: Disturbances to the Earth’s magnetic field can lead birds astray – a phenomenon scientists call “vagrancy” – even in perfect weather, and particularly during fall migration. The research is published in Scientific reports.
The Earth’s magnetic field, which extends between the North and South poles, is generated by several factors, both above and below the planet’s surface. Decades of laboratory research suggests that birds can detect magnetic fields using magnetoreceptors located in their eyes. The new UCLA study supports these findings from an ecological perspective.
“There is growing evidence that birds can actually see geomagnetic fields,” said Morgan Tingley, corresponding author of the paper and associate professor of ecology and evolutionary biology at UCLA. “In familiar areas, birds can navigate by geography, but in some situations it’s easier to use geomagnetism.”
But the ability of birds to navigate using geomagnetic fields can be impaired when these magnetic fields are disturbed. Such disturbances can come for example from the solar magnetic field, in particular during periods of intense solar activity, such as sunspots and solar flares, but also from other sources.
“If the geomagnetic field is disturbed, it’s like using a distorted map that causes birds to veer off course,” Tingley said.
Lead researcher Benjamin Tonelli, a doctoral student at UCLA, worked with Tingley and postdoctoral researcher Casey Youngflesh to compare data from 2.2 million birds, representing 152 species, that had been captured and released between 1960 and 2019 – as part of a United States Geological Survey monitoring program. — against historical records of geomagnetic disturbances and solar activity.
While other factors such as weather likely play a larger role in vagrancy, the researchers found a strong correlation between birds captured well outside their expected range and geomagnetic disturbances that occurred during fall migrations and spring. But the relationship was particularly pronounced during fall migration, the authors note.
Geomagnetic disturbances affected the navigation of both young birds and their elders, suggesting that birds rely equally on geomagnetism regardless of their level of migration experience.
The researchers expected that geomagnetic disturbances associated with increased solar activity would be associated with the greater wandering. To their surprise, solar activity actually reduced the incidence of wandering. One possible reason is that radio frequency activity generated by solar disturbances could render the birds’ magnetoreceptors unusable, leaving them to navigate by other signals.
“We think the combination of high solar activity and geomagnetic disturbances leads to either a pause in migration or a shift to other signals during fall migration,” Tonelli said. “Interestingly, birds that migrate during the day were generally an exception 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 said.
To make the research more accessible to the birding public, Tonelli developed a web tool that tracks geomagnetic conditions and predicts vagrancy in real time. The tracker is offline during the winter, but will be operational again in the spring when the migration begins again.
Thanks to the University of California, Los Angeles for this news.
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