[科学美国人60秒] SSS 2015-10-13（在线收听）

 If you are lost, you need a map and a compass. The map pinpoints where you are and the compass orients you in the right direction. Migratory birds, on the other hand, can traverse  entire hemispheres and end up just a couple of miles from where they bred last year, using their senses alone. Their compass is the sun, the stars and the earth's magnetic fields, but their map is a little more mysterious. One theory goes that they use all factual cues, how a place smells. Another is that they rely on the sense of magnetism. Researchers in Russia investigated the map issue in a past study by capturing ? on the Boltic sea as they flew northeast towards their breeding grounds near St Petersburg. And they moved the birds six hundred miles east near Moscow, and the birds, they just reoriented themselves to the northwest, correctly determing their new position. Now the same scientists have repeated that experiment. Only this time they didn't move the birds at all. They just put them in cages that simulate the magnetic field of Moscow while still allowing the birds to experience the sun, the stars and the smell of the Boltic. Once again the birds reoriented themselves to the northwest, suggesting that the magnetic field alone, regardless of smells and other cues is  enough to author the birds' mental map. The study is in the journal Current Biology. And if you are envious of that sixth sense, keep in mind that since the earth's magnetic field fluctuates, the researchers say magnetic root finding is best for crude navigation, meaning for door-to-door directions, you are still better off with your GPS.