The birds were getting restless. It was 2018 and fall was in the air, triggering their instinct to leave Austria for sub-Saharan Africa. Problem was, these Eurasian Reed Warblers were somehow starting out in entirely the wrong place, in Neftekamsk, Russia—or so they thought.
In reality, the birds were at the Biological Station Lake Neusiedl in Illmitz, Austria, very close to their summer nesting grounds and more than 1,600 miles from Neftekamsk. An international team of researchers had exposed the reed warblers to an electrical field that mimicked the geomagnetic signature of the Russian city to see how they would respond. Apparently convinced by the scientists’ trick, the birds tried to fly back toward their typical migratory route.
The study, out last month in Current Biology, provides the first direct evidence that migratory birds can use Earth’s magnetic field to extrapolate their position and get back on course, even when they are blown far afield. “It shows what an important cue this magnetic field must be to them,” says Richard Holland, an animal behavior researcher with Bangor University in Wales and co-author of the study. “Despite everything else still telling them they’re on their normal route, they react as though they’ve been displaced two and a half thousand kilometers to the northeast.”
Produced by the planet’s rotation and its flowing, liquid-metal outer core, Earth’s magnetic field helps to make life possible by protecting us from solar radiation. Humans have used compasses for hundreds of years to harness the geomagnetic field for navigation, but many other species seem to be able to sense it innately. It’s a handy thing to be able to detect, varying across the Earth’s surface such that different locations have their own magnetic signatures. “So, you’ve got this really neat Cartesian coordinate map, which is kind of the theoretical construct behind what we think the birds are doing,” Holland says.
Scientists have known for decades that Earth’s magnetic field plays a role in bird migration. In 1971, a researcher named William Keeton published a study in which he glued magnets to the backs of homing pigeons and found that the birds became disoriented under overcast conditions, suggesting that their magnetic sense was important for navigating, at least when the sun wasn’t visible. A year later, the husband-and-wife team of Wolfgang and Roswitha Wiltschko published a paper showing that European Robins kept inside without visual cues to their location still tried to take off in the direction of their wintering ground in Africa. When the robins were surrounded by electromagnetic coils that disrupted their sense of the Earth’s magnetic field, however, they got disoriented just as Keeton’s pigeons had.
In the intervening decades, researchers have tried to better understand just how important geomagnetic cues are for avian navigation and how birds detect them in the first place. They have found that birds in their first year of migration seem not yet to have a magnetic map; first-year migrants will keep trying to fly in the same direction even when exposed to confusing magnetic coils or displaced to an entirely different location. “They have a direction that they head in, and some sort of time that they fly for, and that’s it,” Holland says.
That finding indicates that adult birds use experience to build their magnetic mental maps of the world, but how they “see” Earth’s magnetic field is still up for debate. Some researchers have suggested that magnetically sensitive minerals in the beak might play a role, but a 2012 study deflated that hypothesis by showing these were just iron-rich white blood cells. An experiment published in 2018 turned the attention to birds’ eyes, pointing to a special protein that is extra-sensitive to the blue wavelength of light—a wavelength previously linked to birds’ ability to detect magnetic fields. Whatever the mechanism may be, it is increasingly clear that magnetic fields play an important role in how birds find their way.
The new study by Holland and his colleagues further tests the importance of magnetic fields relative to other cues birds might use for navigation. Using mist-nets, the scientists caught adult reed warblers in Austria at the onset of their fall migration and kept them in outdoor aviaries. They were careful to ensure that the birds had full access to the familiar sights and sounds of Austria throughout the study.
When the reed warblers began to show signs of migratory restlessness (also called zugunruhe), it was time for the moment of truth. To discern which direction the birds were attempting to fly, the researchers placed them in a cone-shaped container called an Emlen funnel. These tools, often used to study bird orientation during migration, have a film coating on the inside of the funnel to record the scratch marks left by birds’ feet as they attempt to scramble out.
The researchers first tested the 24 birds in the study in Emlen funnels that allowed them to feel the Earth’s magnetic field. They found that all of the birds oriented in the proper direction for their usual fall migration. Then, the researchers tested 15 of the birds in funnels surrounded by a magnetic coil that mimicked the Earth’s geomagnetic signature in Neftekamsk. The team chose the city because it fulfilled two important criteria: It is far from anywhere the birds would naturally go, and requires them to fly in a direction that was obviously different from their typical flightpath. The reed warblers that thought they were in Russia scrambled southwest, back toward Austria—evidence that geomagnetism is key for birds to understand their relative location and get back on track when blown off course.
While previous studies showed that the magnetic field provides birds with a compass, the new findings suggest that it also gives them a kind of GPS, according to Holland. “If you’ve only got a compass, that tells you a direction, but if you don’t know where you are in relation to where you’re needing to get to, that compass is useless,” he says. The new research “really extends the potential range of the magnetic field as a cue for navigation.”
There might, however, be explanations that don’t require such a sophisticated mental map, cautions Verner Bingman, an animal navigation researcher with Bowling Green State University who was not involved in the study. The birds might fly southwest automatically anytime they get confused, says Bingman, who would like to see follow-up research to understand if they can reorient from other directions as well.
Holland and his team also want to learn more on this front and plan to further study how the reed warblers place themselves in the world and to test the limits of this sense. “It’s difficult to get your head around what’s going on in the bird’s head,” he says. “The fact that these birds have the ability to accurately calculate that they have been displaced, and correct for that, is quite a remarkable thing.”