Off the map

Landmark study finally decodes how birds use Earth's magnetic field as a map

“That is about as close to magic as is possible.”

 standing on branch acrocephalus scirpaceus

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Birds don’t carry smartphones under their wings. When they migrate, GPS isn’t telling them exactly when to turn left and right, or when they’ve “arrived” at their final destination. Instead, they navigate using a perplexing set of innate skills. The Earth’s magnetic field plays a crucial role in this process, guiding them in the right direction. But what truly puzzles researchers is how the birds know when it’s time to stop.

“That is about as close to magic as is possible,” says first author Joe Wynn, who conducted this research as a doctoral student at Oxford University. Wynn is now a postdoc at the Institute of Avian Research in Wilhelmshaven in Germany doing unrelated work.

What’s new — In a paper recently published in the journal Science, researchers found a specific aspect of the Earth’s magnetic field is key to informing a bird’s travel plans.

Scientists already knew there are three components of our planet’s magnetic field that play a role in helping birds navigate during their migrations: Intensity, which is the magnetic field’s strength; declination, or the angle between the Earth’s magnetic North (which varies with the magnetic field, and is what compass needles point to) and true North (a fixed point); and finally, inclination, which is the angle between the Earth’s magnetic field and its surface.

But one thing that has puzzled scientists is that Earth’s magnetic field shifts slightly every year. If the field guides the birds, then they should end up in a slightly different location every migration season; and yet, they return to the same places with baffling precision.

Why it matters — Understanding how and why birds migrate over stunning distances every year can help us unlock the secrets of our animal brethren. But it can also reveal new strategies for conservation.

Bird species are in decline across the world, and new discoveries like this one can not only deepen our understanding and appreciation of birds but also help protect them by safeguarding their migration routes.

Digging into the details — Wynn looked at almost 80 years of data on nearly 18,000 Eurasian reed warblers (Acrocephalus scirpaceus). Wynn calls this bird a “model organism” in that we know nearly everything about it in the same way we know fruit flies and mice so well. These warblers migrate between Wales and sub-Saharan Africa each year, so their journey is substantial.

The team tried to predict where migrating warblers would end up based on those three different ephemeral qualities of the magnetic field and found that predictions based on inclination significantly correlated with where the birds ended up.

What’s even more fascinating is that as predictors of where the birds ended up, intensity and declination were about as effective as flipping a coin. Predictions based on inclination, however, were almost always correct.

Reed warblers may be able to “see” the Earth’s magnetic field lines. Chaithanya Krishnan/Moment/Getty Images

But how do birds sense magnetic inclination?

“The answer is almost unbelievable,” says Wynn. There’s evidence that warblers can see the magnetic field.

In birds’ eyes, a light-dependent reaction yields a chemical that’s proportional to the environment’s active magnetic fields. As the birds move their heads through the field, the chemical reaction would fluctuate. This chemical reaction in the eyes sends a signal to the brain to interpret it. And previous research found that the sensor is located in the visual system of the birds’ brains.

“It’s not unreasonable to suggest birds can see the Earth’s magnetic field,” he says.

Birds aren’t even the only animals to travel by magnetic field. Sea turtles and fish also take cues from these invisible forces.

What’s next — This finding comes with two related caveats: It’s correlative, and it comes from an uncontrolled environment.

One plus of using decades of stored data from real migrations is that it’s the exact conditions, no simulations. But, no simulation means no control over variables, and therefore it’s much harder to attribute cause and effect.

“What this lacks is laboratory experimentation where you can precisely change things and attribute changes, therefore, very precisely to the treatment,” Wynn says.

The most Wynn can do is observe the correlation between his hypothesis and the outcome. He hopes others in his field will continue studying how birds use forces invisible to us to travel such incredible distances.

Abstract: Although it is known that birds can navigate back to their breeding grounds with remarkable precision, it has remained a mystery how they know when and where to stop migrating. Here, using nearly a century of Eurasian reed warbler (Acrocephalus scirpaceus) ringing recoveries, we investigated whether fluctuations in the Earth’s magnetic field predict variation in the sites to which birds return. Ringing recoveries suggest that magnetic inclination is learned before departure and is subsequently used as a uni-coordinate ‘stop sign’ when relocating the natal/breeding site. However, many locations have the same inclination angle. Data from populations with different migratory directions indicate that birds solve this ambiguity by stopping at the first place where the right inclination is encountered on the bird’s inherited return vector.
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