To learn how migrating songbirds keep to a heading on overcast nights, when neither sun nor stars are visible, researchers raised European robins in cages and exposed them, during the restless migratory season, to an artificial magnetic field they could rotate at will. When the field pointed north, the caged birds fluttered and hopped toward the north wall; when the researchers silently rotated the field ninety degrees, the birds shifted their efforts the same amount, though nothing in the room had visibly moved. This suggested the robins were reading the magnetic field directly. A further result narrowed the mechanism. Birds tested in total darkness lost their bearings entirely, orienting at random, but birds given even dim blue light recovered their sense of direction; red light did not help. Because the response depended on light of particular colors, the researchers concluded that the birds' magnetic sense was not a simple internal compass needle but a process seated in the eye, one requiring light of the right wavelength to function. The magnetic field, in other words, was not merely felt. In some fashion the robins had to see it.