Need to train them in nasty weather when they are young. Birds use magnetic compass under these conditions. Show with experiments where magnetic field is messed up. E. The map sense. So birds can use sun and magnetic field for compass, but bees do this, too.
Birds differ because they also have a map sense.
Contact lenses put on homing pigeons which distorted their sight. Birds homed normally until they were in the vicinity of their loft. Then they either wandered around the vicinity or landed awkwardly and walked home. These experiments showed that the birds knew the position of home to within 5km. At this point they used visual cues to find the loft.