But how? Two hypotheses have dominated. One centres on the discovery that birds (and other organisms, including salmon) make and store in their bodies a version of iron called magnetite, which orients itself to magnetic fields.

In birds, magnetite is often concentrated in the beak. Studies have shown that when the beaks of these birds are exposed to powerful magnetic fields—or are numbed with an anesthetic—the birds lose their ability to navigate properly.

But many scientists have suspected that another mechanism is also crucial—one that can tell a bird not only which way is north but also how far it is from the equator by detecting the angle of magnetic field lines. Those lines emerge from Earth’s magnetic poles perpendicular to the planet’s surface, then arch overhead to meet over the equator, at which point they run parallel to the surface. If a bird could detect the angle of those lines relative to the surface, it could know, in effect, its latitude.

Scientists had theorised that a molecule with the right characteristics might change its behaviour depending on the inclination of the magnetic field around it. It might react with another chemical more quickly, for example. In the new work—conducted in a chamber that blocks Earth’s magnetic field and creates fresh ones of various strengths—the team made a three-part molecule that, in response to light, gives up electrons at one end and passes them to the other end. There they linger for a millionth of a second or so before returning. Significantly, the precise amount of time each electron spends in its temporary home at the far end of the molecule varies with the angle of the surrounding magnetic field.

... contd.