How do homing pigeons know which way to fly?

Homing pigeons know where to fly because of a strong homing instinct and their ability to sense magnetic fields, so they can return from over 500 km away, and excellent ones from about 1,000 km. But sending a message is not simply pointing them in a direction; usually when a person goes to deliver a letter, they take several pigeons they already keep and use them to fly the reply back. Not all pigeons have this ability, ancient people selected fine birds and trained them into homing pigeons, sometimes using 2 to 3 at once to ensure delivery.