How do migrating birds know which direction to fly?

Birds use multiple navigation systems simultaneously. Their primary tool is an internal magnetic compass that detects Earth's magnetic field, likely through cryptochrome proteins in their eyes or magnetite crystals in their beaks, allowing them to sense both direction and latitude. They also navigate by the sun during the day, using an internal clock to compensate for its movement across the sky, and by star patterns at night, particularly the rotation point around Polaris. Beyond these, birds rely on landmarks like coastlines, rivers, and mountain ranges, as well as infrasound, smells, and even wind patterns. Much of this ability is innate, with young birds hatching with a genetically encoded migratory direction, which they then refine through experience over successive migrations.