The Compass in the Dark

Every autumn, billions of birds launch themselves into skies that hold no visible path. The European robin, barely larger than a child's fist, leaves its summer home in Scandinavia and flies south to warmer coasts — sometimes crossing open ocean for hundreds of miles in the dark. It has no GPS, no map, no star chart on cloudy nights. What guides it is something it cannot see with ordinary eyes, and yet — according to quantum biologists Henrik Mouritsen at the University of Oldenburg and Thorsten Ritz at UC Irvine — the robin literally perceives Earth's magnetic field as a visual gradient overlaid on its vision.…