The Hidden Corridors of the Sky

Many migratory birds, while moving at speeds of 20–60 km/h, use wind currents at altitudes from hundreds of meters to several thousand meters; it is thought that they can also detect small directional differences in Earth’s magnetic field when choosing their routes. We cannot see this invisible path directly, but we notice its trace in the sudden turning of leaves all in the same direction, in the drift of clouds, and in the line of flocks silently cutting through the sky. If we could see it, the atmosphere might shimmer like a cold and invisible map of routes opening before the birds.