A team led by immunologist Christian Kurts at the University of Bonn and animal behaviour scientist Martin Wikelski at the Max Planck Institute of Animal Behavior has found that iron-laden macrophages in pigeons' livers act as magnetic receptors, according to a study published in Science. The discovery explains how homing pigeons navigate using Earth's magnetic field when sun, landmarks, or smells are unavailable — a puzzle that has baffled biologists for decades.

From mouse spleens to pigeon livers: the accidental path

As reported by the researchers, the breakthrough originated from earlier work in mice, where Kurts observed that immune cells in the spleen accumulate iron and become magnetically sensitive. The team then wondered whether similar cells existed in birds. After screening multiple organs , they found the highest iron concentration not in the spleen,but in the liver, where macrophages — cells that normally recycle old red blood cells — were densely packed near nerve fibres , suggesting a direct route to transmit magnetic signals to the brain.

A drug called clodronate: disabling the compass

To confirm the link,the scientists temporarily depleted macrophages in a group of pigeons using a drug called clodronate. When released under overcast skies — conditions that force reliance on magnetic cues — the treated pigeons became disoriented and flew in random directions, while untreated control birds oriented correctly.. The experiment, conducted by the Max Planck Institute, provides direct causal evidence that liver macrophages are crucial for magnetic sensing, the study states.

What the overcast-sky experiment revealed about navigation

The test under overcast conditions was critical, according to the researchers, because cloud cover eliminates visual cues from the sun, forcing pigeons to use their magnetic sense. Untreated pigeons still found home; treated ones did not. This strongly suggests that the iron-rich macrophages are the primary magnetic receptor, rather than iron particles in the beak or light-sensitive molecules in the eyes, which had been the leading hypotheses for years.

Could fish, whales, and other migrants share the same sense?

David Bird , an emeritus professor of wildlife biology at McGill University, told the researchers the finding is “amazing” and speculated that similar mechanisms might exist in other migratory animals, according to the study. The report notes that future work could explore whether other birds, fish, or even mammals use iron-rich immune cells for magnetoreception. If confirmed, this would rewrite the understanding of animal orientation and highlight a previously unknown role for immune cells in sensory biology.