Using Earth’s Magnetic Field as a GPS, Migratory Birds Navigate Their Way

Many migratory birds use the Earth's magnetic field as a compass, and others can use information from that field to more or less determine where they are on their mental map.

Greater Reed Warbler (Acrocephalus skillupaceus) appears to calculate geographic location by drawing data from various distances and angles between the magnetic field and the shape of the Earth. The study suggests that birds use magnetic information as a kind of “GPS,” telling them not only where to go, but also their initial whereabouts, they said. richard holland At Bangor University, UK.

“When we travel, we have a map that shows us where we are and a compass that shows us which direction to go to reach our destination,” he says. “We don't expect birds to have this much precision or knowledge about the entire planet. Yet, when they travel along their normal path, or even when they travel far from that path, they , and observe how the magnetic cues change.”

Scientists have known for decades that migratory birds rely on cues from the ocean. solar, star and earth's magnetic field To decide which direction to go. But using a compass to figure out direction and knowing where a bird is in the world are markedly different, and scientists are wondering if and how birds figure out their current map location. I'm still debating whether to do it or not.

Florian Packmore Germany's Lower Saxony Wadden Sea National Park Administration suspected that birds could detect detailed aspects of magnetic fields to determine their global location. Specifically, magnetic obliquity (the change in the angle of the Earth's surface relative to magnetic field lines) and magnetic declination (the difference in orientation between the geographic and magnetic poles) are used to better understand where you are in the world. He thought he might be able to do it.

To test their theory, Packmore, Holland and colleagues captured 21 adult reed warblers in Illmitz, Austria, on their migration route from Europe to Africa. So the researchers temporarily placed the birds in an outdoor aviary, where they used a Helmholtz coil to disrupt the magnetic field. They artificially altered the inclination and declination in a way that corresponded to the location of Neftekamsk, Russia, 2,600 kilometers away. “That's way off course for them,” Packmore says.

The researchers then placed the birds in special cages to study their migratory instincts and asked two independent researchers, who were unaware of changes in the magnetic field, to record which direction the birds headed. In the changed magnetic field conditions, most birds showed a clear tendency to fly west-southwest, as if trying to return to their migratory route from Russia. In contrast, when the magnetic field was unchanged, the same birds attempted to fly south-southeast from Austria.

This suggests that the birds believed they were no longer in Austria, but Russia, based solely on magnetic inclination and declination, Packmore said.

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

However, the neurological mechanisms that allow birds to sense these aspects of the Earth's magnetic field are still not fully understood.

“This is an important step in understanding how the magnetic maps of songbirds, especially the great reed warbler, work,” he says. Nikita Chernetsov The professor at the Institute of Zoology of the Russian Academy of Sciences in St. Petersburg was not involved in the study.

The study confirms that the great reed warbler relies on these magnetic fields for positioning, but that doesn't mean all birds do, he added. “Not all birds work the same.”

Packmore and Holland said the birds were released two to three weeks after the study, at which point they were able to continue their normal migration. In fact, one of the birds they studied was captured a second time a year later. This means that the researchers' work did not interfere with the birds' successful migration.