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Humans have used homing pigeons to carry messages across vast distances since ancient times. They delivered military intelligence during wars, business communications before telegraphs existed, and even love letters between separated families.
Yet despite our long history with these remarkable birds, one mystery has remained unsolved: how do pigeons always know where home is?
Scientists have known for decades that pigeons use a combination of clues to navigate. They can recognize landmarks, use the position of the Sun, and even detect smells carried by the wind.
But researchers have long suspected there was another navigation system at work, one that allows pigeons to find their way even when the sky is completely overcast.
A new study published in the journal Science may have uncovered the answer, and it's stranger than anyone expected.
The compass might be hiding inside the pigeon's liver.
Many animals appear capable of sensing Earth's magnetic field.
Migratory birds travel thousands of kilometres with astonishing accuracy. Sea turtles return to the beaches where they were born. Sharks navigate across entire oceans.
Scientists call this ability magnetoreception, the capacity to detect Earth's magnetic field and use it like a compass.
The problem is that nobody has been entirely sure how animals do it.
Now researchers have identified something unexpected: specialised immune cells called macrophages in pigeon livers that contain large amounts of iron.
Normally, macrophages help clean up the body by removing old red blood cells. Because red blood cells contain iron, these liver macrophages gradually accumulate iron-rich particles.
What surprised researchers was that these particles appear to have a property called superparamagnetism.
The team discovered that these iron-rich macrophages sit close to nerve fibres inside the liver.
That means they may be able to detect changes in Earth's magnetic field and pass that information directly to the brain.
To test the theory, the researchers trained 34 homing pigeons to return home from a location 19 kilometres away.
Half of the birds were then given a treatment that temporarily removed these specialised macrophages from their livers.
The next day, the pigeons were released under completely overcast skies.
Every pigeon with an intact liver compass found its way home within about an hour.
The pigeons whose macrophages had been removed became hopelessly disoriented. They flew in random directions and none returned home that day.
When the Sun came out again, those same pigeons were suddenly able to navigate normally.
Their flight ability hadn't been affected. They weren't sick. They simply seemed unable to determine direction when they couldn't see the Sun.
The study suggests that pigeons may possess two different navigation systems.
The first relies on familiar cues such as the Sun and visual landmarks.
The second appears to rely on magnetic information gathered by specialised immune cells in the liver.
When one system becomes unavailable, the other takes over.
If confirmed, this would be one of the most surprising examples yet of how biology repurposes ordinary cells for extraordinary tasks.
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