A study in the journal Science argues that homing pigeons may read Earth's magnetic field using iron-rich cells in their livers, rather than their beaks or eyes. Researchers wiped out those cells in 34 trained birds; under heavy overcast, none could find their way home, while untreated birds all made it back within 70 minutes. Once the sun came out, the treated birds homed normally.
The cells turn magnetic by recycling worn-out red blood cells and hoarding the iron as ferritin. "On cloudy days when they could not see the sun, pigeons whose magnetic macrophages had been depleted were unable to navigate home," the journal's editor notes; those liver cells, the authors conclude, are "required for finding magnetic direction."
The same setup, they add, could explain how bats and blind mole rats navigate without light.
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