WHETHER migratory animals can determine their global position by detecting features of the Earth's magnetic field has long been debated1–4. To do this an animal must perceive (at least) two distinct magnetic parameters, each of which must vary in a different direction across the Earth's surface3,5. There has been no evidence that any animal can perceive two such magnetic features, and whether 'magnetic maps' exist at all has remained controversial2–6. Several populations of sea turtles7–9 undergo transoceanic migrations before returning to nest on or near the same beaches where they themselves hatched. Along the migratory routes, all or most locations have unique combinations of magnetic field intensity and field line inclination. It has been demonstrated that hatchling loggerhead turtles can distinguish between different magnetic inclination angles10. Here we report that turtles can also distinguish between different field intensities found along their migratory route. Thus sea turtles possess the minimal sensory abilities necessary to approximate global position using a bicoordinate magnetic map.