How do migratory birds find their way?

Every year migratory birds fly thousands of kilometers to find food. But, researchers do not know how these birds find their way and travel to the same location year after year. Some researchers suggests that the migratory birds travel along with the earth magnetic lines that emerge from the south pole and merge at the north pole. Also, the migratory birds travel long distance by creating the mental map of their route by remembering mountains, rivers etc.,Besides this, it is also believed that the migratory birds use the location of sun and stars to find their way and reach the same location.