Day or night, rain or shine, wherever they're released, trained pigeons can find their way home over distances as high as almost 1,000 kilometers (around 600 miles).

It's a skill humans have made use of since time immemorial. And for around a century, scientists have known that magnetoreception plays a part in the birds' navigational cocktail. 

A research team from the University of Bonn and the Max Planck Institute of Animal Behavior wrote in the journal Science this week that a core part of the secret to homing using magnetic fields may lie in the birds' livers.

What did the study find? 

The study notes how the "mechanisms underlying" the reception of magnetic information in the rock dove — or Columba livia — "appear to be numerous and are still being discovered."

Some physical connections in more intuitive locations — like the beak, eyes and brain — have already been identified. 

"Here, we used physical, morphological, functional, and genomic assays to identify the presence of superparamagnetic macrophages in the liver," the researchers write. 

Macrophages are immune cells that break down old red blood cells. As part of this process, they accumulate iron, which may allow them to respond to magnetic fields.

"We found that after macrophage depletion, pigeons flying under overcast conditions lacked their usual orientation capabilities," they said. 

When the sun was visible, the birds' orientation was unimpaired, suggesting that visual and solar-based cues were another of the pigeons' navigation methods. 

But given the birds' inability to navigate magnetically without the assistance from their livers, the researchers concluded: "We propose that in homing pigeons, superparamagnetic macrophages in the liver are required for finding magnetic direction."

What did the researchers say? 

Martin Wikelski, Director at the Max Planck Institute of Animal Behavior said that "what looks like a 'gut feeling' in bird navigation may actually have a physical basis." 

The immune cells in question are located near nerve cells in the liver. The researchers postulate that this may be how they can transmit data to the pigeons' brains.

"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," said co-senior author of the study Christian Kurts, Director at the Institute of Molecular Medicine and Experimental Immunology at the University Hospital Bonn.

Despite their surprise about the answer apparently lying in macrophages, the scientists knew ahead of the study that some organs were particularly worthy of study.

"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," said first author Clivia Lisowski, from the University of Bonn and the University Hospital Bonn, who led the immunological work.

How homing pigeons' skills have been known and exploited for millennia

It's unclear when or how it all began, but humans have utilized pigeons' homing abilities since antiquity. 

The birds and their navigational skills feature across ancient cultures, whether it's the Noah's Ark story or in Greek mythology, where the birds were depicted as divine messengers linking the heavens and the earth.

Ancient Greeks used pigeons to deliver news of Olympic winners and victories in battles. The Romans used a pigeon network to accelerate communication across their vast military empire.

Even in the more modern period, as technology began to steal their jobs, pigeons' usefulness endured.

They helped birth the Reuters news agency. In 1850, Julius Reuter in Belgium used pigeons to carry news and stock prices between Brussels and Aachen, Germany because it was still the fastest available means.

Pigeons carried messages from the front lines back to command posts in World War I and were deemed legitimate targets for snipers.

In World War II, British military intelligence's obscure subsection MI14(d) ran Operation Columba, named after the birds' scientific name.

More than 16,000 pigeons previously being used by hobby sports enthusiasts were airdropped into occupied France or other nearby countries in canisters. Locals were urged to fill out an accompanying questionnaire about German troop strength and movements, and other information about their area. They could then release the bird to carry the intelligence back to Britain. The operation gathered actionable intel on German garrison locations, U-boat pens and coastal invasion defenses, among other things.

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Edited by: Karl Sexton