Pigeons may have been carrying messages for thousands of years, and scientists still have not pinned down exactly how they get home. A new study points to an unexpected place, the liver.

Animals use various techniques to navigate, including following the stars and remembering key landmarks. Birds, fish and turtles also orient themselves using Earth’s magnetic field as a compass, but it’s not yet clear how exactly they do this.

Pigeons can traverse hundreds of miles, or hundreds of kilometers, in a single day. Scientists have long tried to untangle how they travel without getting lost. Some think the birds detect magnetic cues using light-sensitive molecules in their eyes, while others suggest it happens in the beak or inner ear.

“The magnetic sense has been this mystery for almost 100 years,” said Martin Wikelski with the Max Planck Institute of Animal Behavior in Germany.

In the new study, Wikelski and other researchers searched for magnetic clues in the birds’ organs and found a strong signal in the liver. Specialized immune cells in the pigeon’s liver break down red blood cells and store iron.

When scientists temporarily stripped pigeons of those immune cells and let them fly, the birds “just couldn’t find their way,” said Christian Kurts with the University of Bonn in Germany.

That suggested the iron-rich liver cells might play a role in their sense of direction. The birds’ magnetic compasses only got scrambled on overcast days, because they also use the sun as a navigational guide.

Scientists have previously wondered if immune cells could be involved in magnetic sensing, but the new study, published Thursday in the journal Science, is the first to present a full-fledged theory.

“I would never have guessed it, but once it was explained to me, it makes sense,” said behavioral ecologist Albert Kao with the University of Massachusetts Boston, who had no role in the study.

The immune cells are located near nerve fibers in the liver. That might be how they transmit their “magnetic sense” to the brain, “and help the pigeons to navigate,” said study co-author Clivia Lisowski with the University of Bonn.

The researchers think other birds and animals, like mice, could operate using a similar magnetic GPS. But outside experts said more work is needed to verify that pigeons navigate this way and to firm up how these signals get to the brain.

While the researchers found the strongest magnetic signal in the pigeons’ livers, such immune cells have also been spotted in other areas, including the beak and spleen. It is possible this magnetic puzzle does not have a single answer, wrote veterinary pathologist Simon Spiro and biologist Hal Drakesmith in an accompanying editorial.

The birds could use different techniques to sense magnetic fields depending on the task, be it traveling long distances or finding a specific destination. “Indeed, it could be prudent to have more than one way of getting home in the dark,” they wrote.

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