Four decades after scientists showed that migratory birds use Earth’s magnetic field to orient themselves during their seasonal journeys, researchers have at last found a molecular mechanism that may explain how they do it.

If the hypothesis is true, the planet’s magnetic field lines — which arch around Earth from north to south — may be plainly visible to birds, like the dashed line in the middle of a road.

The work, described online yesterday in the journal Nature, was conducted in a test tube and does not prove that birds actually use the mechanism. And researchers aligned with a competing model say they are not convinced.

But by identifying for the first time a molecule that reacts to very weak magnetic fields, the experiments prove the plausibility of a long-hypothesized method of avian navigation that has had a credibility problem because no one had ever found a molecule with the required sensitivity.