According to a study published in Science, pigeons may use specialized immune cells in their livers to detect Earth's magnetic field, providing them with an internal navigation system.

Researchers found that these cells, called macrophages, accumulate iron while breaking down old red blood cells. The iron gives the cells unique magnetic properties that could allow them to respond to the planet's magnetic field. When the cells were removed, pigeons struggled to find their way home, pointing to a previously unknown role in navigation.

"We didn't expect immune cells to act like sensors for magnetic fields at all. Our results reveal a previously unknown mechanism for magnetic perception in animals," says Prof. Christian Kurts, Director at the Institute of Molecular Medicine and Experimental Immunology at the University Hospital Bonn, and one of the study's co-senior authors.

"What looks like a 'gut feeling' in bird navigation may actually have a physical basis," adds Prof. Martin Wikelski, Director at the Max Planck Institute of Animal Behavior and the other co-senior author of the study.

The Long Search for Birds' Magnetic Sense

Scientists have long known that homing pigeons and migratory birds use Earth's magnetic field as one of several tools for navigation. However, exactly how animals detect that field has remained one of biology's biggest mysteries.

Over the years, researchers proposed several possibilities. Some theories suggested birds could detect magnetic fields through light-sensitive molecules in their eyes. Others pointed to tiny magnetic particles in their beaks. Despite years of investigation, neither idea has received strong experimental confirmation.

The new study offers a different explanation, combining expertise from immunology, physics, and animal behavior. The research team included scientists from the University of Bonn, the University Hospital Bonn, the University of Duisburg-Essen, and the Max Planck Institute of Animal Behavior (MPI-AB).

Iron-Rich Liver Cells Show Strong Magnetic Properties

To determine where magnetic sensing might occur, the researchers examined multiple organs that have previously been linked to magnetoreception, including the eyes, beak, and brain. They also analyzed the liver and spleen using techniques known as "vibrating sample magnetometry" and "magnetic cell separation."

"We had some clues that the liver and spleen have magnetic properties, because they break down red blood cells and so store much iron in the body," says first author Dr. Clivia Lisowski, from the University of Bonn and the University Hospital Bonn, who led the immunological work.

The results were striking. Among all the tissues studied, the liver contained the highest concentration of iron and produced the strongest magnetic response.

"Iron is crystallized in oxide nanoparticles making the cells superparamagnetic and reactive to magnetic fields. We found by far the strongest magnetic response in liver tissue," adds Prof. Ulf Wiedwald, from the University of Duisburg-Essen.

Further investigation revealed that liver macrophages were responsible for these magnetic properties.

Navigation Experiments Reveal a Critical Role

The researchers then tested whether the macrophages actually influence navigation.

At the MPI-AB in Konstanz, Germany, pigeons had been trained to return to their aviary from locations more than twenty kilometers away. Scientists removed the liver macrophages and monitored how the birds performed.

The results depended on the weather. On overcast days, when the sun was hidden, pigeons that lacked the macrophages lost their sense of direction and had difficulty navigating home. On sunny days, however, they successfully returned, likely relying on the sun as a navigational cue instead of Earth's magnetic field.

These findings suggest that birds use magnetic information alongside solar cues to orient themselves during flight.

How Magnetic Signals May Reach the Brain

After establishing a link between the liver cells and navigation, the researchers looked for a way the information could travel to the brain.

Using electron microscopy, they found that the iron-rich macrophages sit close to nerve fibers. This arrangement suggests a possible pathway through which magnetic information could be transmitted from the liver to the nervous system and ultimately to the brain.

Lisowski says: "These findings provide the first concrete evidence of how the Earth's magnetic field can be perceived within the body and passed on to the brain to guide movement."

The study brings together several well-established biological processes, including iron metabolism and communication between the immune and nervous systems, to help explain how animals may detect magnetic fields.

"Animal navigation is one of the most fascinating phenomena in nature," says Wikelski. "If immune cells are part of how birds sense direction, it would fundamentally change how we understand navigation."

Implications Beyond Birds

Although the findings answer important questions, many remain. Researchers still need to determine exactly how the brain processes signals coming from these cells.

The discovery could also have broader implications beyond pigeons. Animals such as sharks are known to navigate effectively without relying on light, raising the possibility that similar mechanisms could exist in other species.

The researchers suggest that many animals, and perhaps even humans, may respond to magnetic fields in ways that are not yet fully understood.