Researchers recently showed that migratory reed warblers depend on an internal geomagnetic map to guide them on their long-distance journeys. But it wasn’t clear how the birds were solving the relatively difficult “longitude problem,” determining where they were along the east-west axis and which way to go. Now, the team’s latest report published in Current Biology on August 17 has an answer. The birds rely on changes from east to west in magnetic declination, the angular difference between geographic north and magnetic north.