Ley Lines are magnetic lines that criss-cross the Earth. Early humans may have been able to detect them, since many prehistoric monuments, such as Stonehenge, are built along them. Migratory birds may be able to see them and use them as a map.
“A fascinating possibility is that [birds] may actually see the earth’s magnetic lines as patterns of color or light intensity superimposed on their visual surroundings,” says biologist John B. Phillips. “Some animals can see ultraviolet light. Some animals can see polarized light.” It’s now known that bees can see “landing strip” patterns on flowers that are invisible to human eyes.