Get the Popular Science daily newsletter💡 Breakthroughs, discoveries, and DIY tips sent six days a week. By signing up, you confirm you are 16+, will receive newsletters and promotional content and agree to our Terms of Use and acknowledge the data practices in our Privacy Policy . You may unsubscribe at any time. For decades, scientists have known that Earth’s magnetic field helps migratory birds and homing pigeons navigate. Just how our feathered friends sense the invisible sphere around the Earth, however, has been less clear.  At least part of the answer appears to be hiding inside a seemingly random organ. Immune cells inside pigeon livers called macrophages are sensitive to the planet’s magnetic field . These cells function like an internal compass, according to a new study published today in the journal Science .  Macrophages destroy old red blood cells, which makes them accumulate iron.…