By 
A world concealed from man but known to birds — Science News, April 13, 1974
What subtle sensory cues enable a bird to navigate … with spectacular accuracy? [Researchers] have found that birds can sense small changes in air pressure … and can “see” polarized light …. Meanwhile, biologists at the State University of New York at Stony Brook have found that by attaching a pair of small coils around the heads of homing pigeons and altering an applied magnetic field, they could change the orientation of the birds’ flight.
Update
Birds appear to navigate using a host of sensory cues — from looking at the position of the sun, stars and landmarks to smelling the air and even detecting Earth’s magnetic field (SN: 7/3/15). Scientists are still unraveling the details of birds’ magnetic sense. Iron-rich cells within the beak have long been suspected to function as internal compasses (SN: 4/3/18). But recent studies have suggested that proteins in the retina called cryptochromes allow birds to “see” Earth’s magnetic field (SN: 6/27/16). In lab tests, cryptochromes from migratory robins were especially responsive to magnetic fields (SN: 6/28/21).
