Eurasian reeds migrate between Europe and Africa
AGAMI Photo Agency / Alamy Stock
Many migratory birds use the Earth’s magnetic field as a compass, but some can also use information from that field to determine more or less where they are on a mental map.
Eurasian reeds (Acrocephalus scirpaceus) calculate their geographic position by plotting data at different angles and distances between magnetic fields and the shape of the Earth. The findings suggest that birds use magnetic information as a kind of “GPS” that tells them not only where to go, but where they are to begin with. Richard Holland at Bangor University in the UK.
“When we travel, we have a map – that tells us where we are – and we have a compass, which way to get to our destination”, he says. “We don’t think birds have that level of accuracy or knowledge of the whole Earth. However, they see how the magnetic signals change as they move from their normal path, or even if they are very far from that path.’
Scientists have known for decades that migratory birds rely on signals the sunhas the stars and Earth’s magnetic field to determine which direction to go. But knowing their direction using a compass is very different from knowing where they are in the world, and scientists still debate how birds know their current map position.
Florian Packmor The Wadden Sea National Park Authority in Lower Saxony, Germany, suspected that specific aspects could be recognized by the birds. magnetic field to determine their global position. Specifically, he thought that magnetic inclination – the changing angle of the earth’s surface with respect to its magnetic lines – and magnetic declination – the difference in direction between the geographic and magnetic poles – could be used to more precisely understand where they are in the world.
To test this theory, Packmor, Holland and their colleagues captured 21 adult reed warblers on their migration route from Europe to Africa in Illmitz, Austria. There, the birds were temporarily placed in outdoor aviaries, where the researchers used a Helmholtz coil to disrupt the magnetic fields. The inclination and declination were artificially changed to correspond to a position 2,600 kilometers away in Neftekamsk, Russia. “That’s out of the way,” says Packmor.
The team then put the birds in a special cage to study their migratory instincts and asked two independent researchers – who were unaware of the changes in the magnetic field – to record the direction the birds went. In the altered magnetic field conditions, most of the birds showed a clear preference to fly from west to southwest, as if they were trying to return to the migration route from Russia. In contrast, the same birds preferred to fly south-southeast of Austria when the magnetic field was unchanged.
This suggests that the birds thought they were no longer in Austria, but Russia, based solely on their magnetic inclination and declination, Packmor says.
“Of course, they don’t know it’s Russia, but it’s too far north and east of where they should be,” Hollande says. “And at that point, they look to their compass system to figure out how to fly south and west.”
However, we still do not fully understand the neurological mechanisms that allow birds to perceive these aspects of the Earth’s magnetic field.
“This is an important step in understanding how magnetic maps of songbirds and, in particular, reeds work,” he says. Nikita Chernetsov at the Zoological Institute of the Russian Academy of Sciences in St. Petersburg, who was not involved in the research.
While the research confirms that reed warblers rely on these magnetic fields to locate themselves, it doesn’t mean that all birds do, he added. “Not all birds work the same way.”
The birds were released two to three weeks after the study, at which point they were able to resume their normal migration, Packmor and Holland said. In fact, one of the birds they studied was captured a second time a year later, meaning that the team’s research did not prevent it from successfully migrating.
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