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In a typical summer, Joe Wynn would travel to a remote island in Wales where for the past few years he has studied how Manx Shearwaters, a type of seabird, navigate from breeding colonies to wintering sites. But in 2020, pandemic restrictions prevented him from visiting the island. Wynn decided not to let that stop his research, however, and found a new bird to fill the gap, one he could study using data already collected—the Eurasian Reed Warbler.

These petite olive-brown birds travel more than 3,000 miles roundtrip between their breeding territories in Europe and wintering areas in sub-Saharan Africa. Earlier research discovered that Eurasian Reed Warblers a combination of cues from Earth’s magnetic field to navigate their southward autumn migrations. For a new study, Wynn and colleagues wanted to know if these same magnetic cues help the birds locate their breeding territories in the spring. The results, , reveal that the birds use magnetic fields differently during their springtime return, relying on a kind of invisible “stop sign” to tell them when they’ve reached their breeding grounds.

If you’ve ever used a GPS, compass, or smartphone, you have taken advantage of Earth’s magnetic field. Almost 2,000 miles below our feet, the planet’s liquid outer core churns as the Earth rotates, generating electrical currents and the magnetic field by which fish, whales, sea turtles, and birds navigate. The qualities of the magnetic field vary across the globe so that specific locations have unique magnetic signatures, but unlike the gridded coordinate map we use to orient ourselves, the magnetic field is dynamic and over years. 

Led by Wynn, a bird biologist at Oxford University at the time, the researchers wanted to understand whether the birds use these signals to know when they have reached their breeding territory. Eurasian Reed Warblers have a high degree of nest-site fidelity, returning each year with remarkable precision to breed in almost the exact same location where they hatched as chicks, says Wynn, making them an excellent species to ask this question.

To find out, Wynn, now a researcher at the Institut für Vogelforschung in Germany, tapped into nearly eight decades of data from bird banding records. Researchers generally place leg bands with unique identifying numbers on songbirds as a way to track population size, but also to record information about birds’ body condition, breeding status, and the timing of arrival to breeding grounds. In this case, the data came from an agency coordinating banding efforts throughout Europe. The trove included nearly 18,000 records of banded birds that were recaptured by scientists.

The researchers then looked at when and where each bird was captured and compared it to three measures of the magnetic field—intensity, declination, and inclination—at that date and location. The intensity, or the strength, of the magnetic field increases from the equator toward the poles. Declination is the difference between magnetic north and true north. And inclination describes the angle at which the magnetic field dips into the Earth’s surface, and generally increases from the South Pole to the North Pole.

Since the magnetic signature of a site changes year to year, researchers would expect a bird’s breeding site to shift along with it—if, that is, the birds pick their breeding sites based on the same combination of magnetic cues the earlier research showed they use in the fall. So, Wynn and team measured the distance between where a bird following those signals would be expected to be captured in a given breeding season, and where the banding records show it actually was captured.

But when they analyzed the data, the researchers were surprised by what they found: The results showed that only magnetic inclination affected their return to their breeding territory; the other cues did not influence their choice of breeding site. Inclination stood out as the main signal even when the researchers accounted for other environmental data, such as temperature. 

Birds inherit from their parents a sense of the direction from their wintering grounds back to their breeding sites. The findings suggest that spring migrants continue along that path until they detect the right inclination angle. That same signal may exist in many locations, but it’s this intersection of magnetic inclination with their inherited migratory route that tells birds when they’re home, says Wynn. 

Richard Holland, an animal behavior researcher with Bangor University in Wales who has done related work but was not involved with the study, explains that it may be inclination’s relative stability that makes it a good stop sign for the birds. While previous work has shown that Eurasian Reed Warblers rely on a combination of magnetic cues for fall migration and to get back on track after a storm blows them off course, intensity and declination might change too much from year to year for the warblers to use them as reliable signs of home come spring. But the inclination angle of where they hatched—which the birds most likely learn before setting out on their first migration—doesn’t change as much, Holland says. “Depending on the task they are doing, different cues might be important.”

Since the 1970s, scientists have known that birds use Earth’s magnetic field to orient themselves—but how birds detect the magnetic field is still uncertain. “That’s the million-dollar question,” says Holland. Scientists suspect that a receptor in the eye, dependent on the amount and color of light, reacts to subtle changes in components of the magnetic field, like inclination. 

While Wynn thinks inclination could act as a stop sign for other migratory songbirds, he speculates that its importance for other species may depend on their ecology. Eurasian Reed Warblers are particular about their nest locations, preferring riparian areas with—you guessed it—plenty of reeds. Other birds that are less picky may not need the magnetic signal to stop at a specific breeding site.

These findings sharpen scientists’ understanding of how birds find their way in the world with such incredible precision, but key questions remain unanswered, Wynn says. And so, for researchers studying the mysteries of bird navigation, there’s no stop sign in sight.