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Many migratory birds use 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 reed warblers (Acrocephalus scirpaceus) appear to calculate their geographical position by drawing data from different distances and angles between magnetic fields and the Earth’s shape. The findings suggest that the birds use magnetic information as a sort of “GPS” that tells them not only where to go, but where they are initially, says at Bangor University in the UK.

“When we travel, we have a map – which tells us where we are – and we have a compass, which tells us which way to go to reach our destination,” he says. “We don’t think birds have quite this level of accuracy or degree of knowledge of the whole Earth. Even so, they see how magnetic cues change as they move along their normal path – or even if they’re far displaced from that path.”

Ů��С��Ƶs have known for decades that migratory birds rely on cues from the , the and to determine which direction to head towards. But figuring out direction using a compass is markedly different from knowing where in the world they are, and scientists still debate about whether – and how – birds figure out their current map position.

at Lower Saxon Wadden Sea National Park Authority in Germany suspected birds could detect detailed aspects of the magnetic field to determine their global position. Specifically, he thought they might use magnetic inclination – the changing angle of Earth’s surface relative to its magnetic lines – and magnetic declination – the difference in direction between the geographic and magnetic poles – to understand more precisely where they are located in the world.

To test that theory, Packmor, Holland and their colleagues captured 21 adult reed warblers on their migration route from Europe to Africa in Illmitz, Austria. There, they placed the birds temporarily in outdoor aviaries, where the researchers used a Helmholtz coil to interfere with magnetic fields. They artificially altered the inclination and declination in a way that corresponded to a position in Neftekamsk, Russia, 2700 kilometres away. “That’s way out of their direction,” says Packmor.

The team then put the birds in a special cage for studying migratory instincts and asked two independent researchers – who were unaware of the changes in magnetic field – to record which way the birds headed. In the modified magnetic field situations, most of the birds showed a clear penchant for flying west-southwest, as though they were trying to return to their migration route from Russia. By contrast, the same birds wanted to fly south-southeast out of Austria when the magnetic field was unmodified.

This suggests that the birds believed that they were no longer in Austria, but in Russia – based on their magnetic inclination and declination alone, says Packmor.

“Of course, they don’t know it’s Russia, but it’s too far north and east of where they should be,” says Holland. “And then at that point, they look at their compass system to work out how to fly south and west.”

However, we still don’t fully understand the neurological mechanisms that enable birds to sense these aspects of Earth’s magnetic field.

“This is an important step in understanding how magnetic maps of songbirds – and in particular, reed warblers – work,” says at the Zoological Institute of the Russian Academy of Sciences in Saint Petersburg, who was not involved in the study.

While the research confirms reed warblers rely on these magnetic fields for positioning, it doesn’t mean that all birds do so, he adds. “Not all birds work the same way.”

The birds were released two to three weeks after the study, at which time they could continue their normal migration, Packmor and Holland say. Indeed, one of the birds they studied was captured a second time a year later, meaning the team’s research did not prevent it from migrating successfully.