In southern Germany, scientists released 34 homing pigeons about 19 kilometers from their home. About half returned within 70 minutes, but the other half became lost and disoriented. The surprising discovery was that the difference had nothing to do with their eyes, brains, or beaks. It was in their livers.
An international team of researchers found that homing pigeons sense Earth's magnetic field using special immune cells called macrophages in their livers. These cells contain iron particles that become magnetized when exposed to the planet's magnetic field. The particles sit next to nerve fibers in the liver, allowing magnetic information to travel to the brain.
To test this theory, the scientists removed these cells from about half the pigeons using a special compound. Under cloudy skies, the pigeons without liver macrophages could not find their way home. However, the pigeons with their cells intact returned successfully. Interestingly, on sunny days, even the depleted pigeons navigated home perfectly well. This shows that pigeons use the sun as their primary guide, but when clouds block the sun, they depend on their magnetic sense in the liver.
This discovery solves a mystery that has puzzled biologists for decades. The immune cells that normally clean up old blood cells in the body also work as a compass, helping these remarkable birds find their way home.