Pigeon livers may steer birds, but skeptics push back

A new study in *Science* argues that homing pigeons use magnetic immune cells in their livers as a compass-like system. But multiple experts say key biophysical steps still aren’t proven—and that earlier work and iron chemistry may undermine the mechanism.

The next time a homing pigeon veers through a gray sky. it might not be guided by what most people think of as a “compass” at all. A new paper in *Science* argues the answer may be hiding inside the birds’ livers—packed with magnetic immune cells that the researchers say can be disrupted with a drug. leaving the birds completely lost.

In the study, the authors report that homing pigeon livers contain magnetic immune cells with a specific form of iron. When those cells were removed, the birds’ navigation broke down. The researchers also describe those iron-bearing macrophages sitting close to nerve endings. hinting at a route for information to reach the brain. But the mechanism—how weak Earth’s magnetic field becomes usable geographic information—remains unresolved. and several scientists say the explanation still doesn’t clear the bar.

Martin Wikelski. a researcher at the Max Planck Institute of Animal Behavior in Radolfzell. Germany. and a co-senior author of the study. said the new hypothesis “really fits all the evidence that’s out there.” He also suggested the idea could be widespread across animals: possibly “happening from bees to mammals and bats to all kinds of birds. and so on.”.

The liver theory comes with an immediate biological hook. The immune cells at the center of the work are macrophages—described by co-senior author Christian Kurts. an immunologist at the University Hospital Bonn in Germany. as the body’s cleanup crew. When a red blood cell dies, Kurts explained, the carcass and its iron can’t just sit and provoke inflammation. Macrophages “empty their trash bags,” recycling the iron back into the bone marrow to build new red blood cells. But before that recycling completes, Kurts said, “the macrophages are full of iron.”.

In humans, these metal-rich macrophages gather mainly in the spleen. To find out where the analogous cells sit in pigeons. the team used an extremely powerful magnetic field—far stronger than Earth’s—to sweep the birds’ bodies. The researchers found the magnetic immune cells in the livers, not just elsewhere in the body. They also observed that the magnetic immune cells abutted nerve endings. raising the possibility of a pathway from cellular magnetism to neural signaling.

To test whether those macrophages mattered for navigation. the team turned to pigeons trained to fly home to their aviary in Radolfzell. Germany. from nearly 20 kilometers away. On a cloudy day—when visual guidance is limited—the researchers gave the pigeons a drug that selected and killed macrophages. Then they watched what happened as the birds attempted to home.

Immunologist Clivia Lisowski described the outcome starkly: “They were completely lost. I mean, it was crazy—they were going in all directions.”

Yet even with that disruption. the paper does not provide an immediate answer to the most important question: how liver macrophages could respond to Earth’s relatively weak magnetic field. and then transmit that information to the brain. Lisowski said the team has ideas, but not proof. “We have our theory, but right now it would just be speculation,” she said.

The doubt isn’t only about speculation. Some experts point to a history of near-misses and conflicting evidence about what kind of iron could plausibly work in a magnetoreception system. A landmark 2012 Nature study. Lisowski’s own work references indirectly through its conclusions. had suggested a previous claim about pigeon navigation being tied to compasslike neurons in the beak was wrong. That study found those cells were iron-bearing macrophages instead. But other past studies concluded that the macrophages carried the wrong kind of iron to explain a magnetic sense. because that form barely responds to Earth’s field—far weaker than the laboratory magnets used to probe such behavior.

In the new paper. the authors try to address that mismatch with “superparamagnetism. ” a quantum mechanical phenomenon described as heightening the iron’s response. But Carl Meyer. a biologist at the Hawaii Institute of Marine Biology in Honolulu who studies magnetic navigation in sharks and was not involved in the research. said there is no evidence that superparamagnetism would be enough for a cell to pick up the signal and notify a neuron. “I remain skeptical—not least because of all the previous declarations of victory,” Meyer said.

Joe Kirschvink. a geobiologist at the California Institute of Technology and not involved in the study. took the critique further back to the peer-review threshold. “I am not convinced,” he said. He added that he was “surprised this paper cleared the review process for Science.” Kirschvink has studied the relationship between Earth’s magnetic field and animals for decades and said that the kind of biophysical models that would make superparamagnetic magnetoreceptors work “basically hit a dead end.”.

Pascal Malkemper, a neuroscientist also not involved in the study, focused on a different snag. What stood out to him wasn’t just the magnetic chemistry—it was that killing the pigeons’ liver macrophages disrupted navigation. But he argued that the findings still don’t establish what caused the birds to fail. Research has shown that pigeons can rely on plenty of cues besides magnetism, including visual and smell-based cues. Malkemper wondered whether the drug might have stirred up the birds in other ways rather than specifically shutting down the magnetic sensor. “The magnetic sense is usually the least important sense somehow. It’s kind of the last resort,” he said. “It’s a lot of correlational evidence and no causality,” he added.

The researchers attempted to reduce the chance that those other cues would carry the navigation anyway. They tested on foggy, cloudy days—conditions meant to blunt landmarks. Malkemper pointed out, though, that the paper offers “a lot of correlational evidence,” not a clean causal chain.

There is another piece of the story that complicates the simple picture. The new paper also found that pigeons treated with the macrophage-killing drug could still find their way home on sunny days. On those days, birds are able to use solar cues to navigate. Kirschvink said that matters: “On sunny days many motivational things are different.”.

Even among the skeptics, there’s a shared respect for the effort to put a new idea on the table. Malkemper said it’s “always good to have a new hypothesis on the table. ” and suggested the research direction fits the broader challenge of magnetoreception. where scientists have looked in the inner ear. in the eye. and now may also consider the liver. “We’re looking in the inner ear. we’re looking in the eye—now we might be looking in the liver as well. ” he said.

For now. the pigeon compass debate hasn’t settled—it has just moved to a new organ. and a new set of unanswered steps. The study offers an elegant biological target: magnetic macrophages in the liver, tied to navigation when the cells are removed. But as long as experts can point to iron chemistry concerns. missing biophysical links. and the possibility that the drug disrupts more than magnetism. the livers-and-compass claim remains a promising trail—not yet a closed case.

homing pigeons magnetic navigation pigeon liver macrophages superparamagnetism magnetoreception iron neuroscience immunology Science journal