Published: June 05, 2026, 13:05 UTC
How homing pigeons navigate across hundreds of kilometers to return to their lofts has puzzled scientists for centuries. They use the sun as a compass when it is visible. They use the Earth’s magnetic field when it is not. But the biological hardware that allows them to sense magnetism has remained one of the most stubborn mysteries in animal behavior.
A new study published May 28 in Science suggests an answer, and it comes from an unexpected place: the immune system.
The research, led by Clivia Lisowski and Christian Kurts of the University of Bonn, Martin Wikelski of the Max Planck Institute of Animal Behavior, and Ulf Wiedwald of the University of Duisburg-Essen, identifies macrophages in the pigeon liver as the source of magnetic sensitivity. Macrophages are white blood cells whose primary job is to engulf and digest pathogens and cellular debris. In the process of breaking down old red blood cells, they accumulate iron in the form of oxide nanoparticles stored in ferritin proteins. These nanoparticles make the cells superparamagnetic, meaning they become strongly magnetized in the presence of even a weak magnetic field.
The Earth’s magnetic field, at 25 to 65 microtesla, is sufficient.
A Truly Interdisciplinary Experiment
The team combined vibrating-sample magnetometry, histology, genomic assays, and GPS tracking of live birds to build the case. They measured the magnetic response of tissue samples from pigeon livers, spleens, muscle, beaks, eyes, and brains. The liver produced by far the strongest signal. Within the liver, the magnetic cells were identified as macrophages.
Histological examination revealed that these macrophages sit in close proximity to, and in some cases in direct contact with, nerve fibers. This anatomical arrangement suggests a plausible route for transmitting magnetic information to the brain, likely via the vagus nerve.
To test whether the macrophages actually contribute to navigation, the team depleted liver macrophages in one group of pigeons and released them under overcast skies 19 kilometers from their loft. Every control bird returned home within 70 minutes. Not a single macrophage-depleted pigeon made it back that day; their GPS tracks showed random drifting in various directions.
The same depleted birds returned normally on sunny days, confirming that the liver-macrophage system is a magnetic compass backup used specifically when the sun compass is unavailable.
A New Layer of Immuno-Sensation
The term “immuno-sensation” has been used in recent years to describe immune cells that detect signals beyond pathogens: tissue damage, metabolic stress, temperature changes. This study extends the concept to magnetic fields. Macrophages, it turns out, are not just cleaners of cellular debris; they may also be compass needles.
“How they sense the Earth’s magnetic field has been a mystery,” Lisowski said. “Several hypotheses were suggested, but these have often resulted from laboratory experiments. They were hardly ever done in the field.”
Previous hypotheses have focused on cryptochrome proteins in the eyes (which use quantum spin dynamics) or magnetite crystals in the beak. The new study does not definitively rule out these mechanisms; they may coexist with the liver-macrophage system. But the behavioral evidence from the depletion experiment is striking.
The team also noted that pigeons display a distinctive circling behavior after take-off. They speculate that this circling may allow the unpaired electrons in ferritin to align with the geomagnetic field, effectively “imprinting” magnetic information into the macrophages before the bird settles into directed flight.
Caveats
Several limitations should be considered. Macrophage depletion is a crude experimental manipulation that affects many biological processes beyond iron storage; the sunny-day control mitigates this concern but does not eliminate it entirely. The mechanism of signal transduction from macrophages to nerve fibers has not been fully characterized, only observed anatomically. The circling behavior hypothesis is speculative and has not been tested directly. The study was conducted in homing pigeons only; whether this mechanism generalizes to other migratory birds or to animals such as sea turtles, sharks, and bats is unknown.
The same type of superparamagnetic macrophages has been found in mice and humans. Their role in magnetic sensing in mammals has not been explored, and any implications for human biology remain speculative.
Still, for a question that has resisted answers for centuries, the study provides the first concrete mechanism with behavioral validation in the field.
“Animal navigation is one of the most fascinating phenomena in nature,” Wikelski said. “If immune cells are part of how birds sense direction, it would fundamentally change how we understand navigation.”
Reference
Lisowski, C., Quetting, M., Klaus, D., Kurts, C., Wikelski, M., Wiedwald, U. et al. “Homing Pigeon Navigation Relies on Superparamagnetic Macrophages Under Overcast Conditions.” Science, Vol. 392, Issue 6801, pp. 985-991 (2026). DOI: 10.1126/science.ady2486