A new study in Science found that immune cells called macrophages in the pigeon liver become magnetic. Macrophages eat up old red blood cells, break them down, and accumulate so much iron that the cell itself starts behaving like tiny magnets. Homing pigeons can find their way home even when the sky is cloudy and the sun is no longer visible as a navigational cue. Scientists have long suspected that pigeons can sense Earth’s magnetic field, but exactly how they do so has rema