https://www.nationalgeographic.com/news/2007/9/birds-can-see-earths-magnetic-field/
The finding strongly supports the hypothesis that migratory birds use their visual system to navigate using the magnetic field.
"The magnetic field or magnetic direction may be perceived as a dark or light spot which lies upon the normal visual field of the bird," Heyers said, "and which, of course, changes when the bird turns its head."
Scientists not involved with the study said it is impressive and well done, but cautioned that there are more pieces to the puzzle of how birds navigate on their long migrations.
"An animal that has to migrate over great distances needs to have both a compass and a map," said Cordula Mora, a biologist who recently completed her postdoctoral research at the University of North Carolina, Chapel Hill.
Mora's work suggests that birds
may use magnetic crystals in their beaks to sense the intensity of the magnetic field and thus glean information on their physical location. (Related news: "Magnetic Beaks Help Birds Navigate, Study Says" [November 24, 2004].)
https://www.forbes.com/sites/trevornace/2018/04/04/we-finally-know-how-birds-can-see-earths-magnetic-field/#14a6987020e1
While scientists have known for quite some time that birds can see Earth's magnetic field, it was
unclear exactly how birds are able to visualize the magnetic field. Two recent studies from researchers at Lund University in Sweden and Carl von Ossietzky University Oldenburg in Germany discovered that the ability is a result of a special protein in bird's eyes. The two papers studied European robins and zebra finches and found evidence for an unusual eye protein called Cry4
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3552369/#!po=86.5079
The magnetite-based receptors in the beak of birds and their role in avian navigation
R. Wiltschko and W. Wiltschko
However, if this compass is disrupted by certain light conditions, migrating birds show ‘fixed direction’ responses to the magnetic field, which originate in the receptors in the beak. Together, these findings point out that there are magnetite-based magnetoreceptors located in the upper beak close to the skin. Their natural function
appears to be recording magnetic intensity
The magnetite-based receptors in the beak, in contrast,
seem to contribute to the processes that determine the course to be pursued, the avian ‘map’ mechanism. In view of this, it
appears most likely that the iron-rich structures mediate information on magnetic intensity.
The hesitance of control pigeons to leave within the anomaly seemed to reflect confusion caused by the irregular magnetic conditions they encountered when flying around. These birds needed some time to realize that the magnetic field was not reliable and finally turn to other, non-magnetic cues. The birds with their beak anesthetized, in contrast, being deprived of the irregular magnetic input, seemed to turn to other cues right away.
Behavioral evidence indicates that there are magnetoreceptors in the beak of birds. These receptors include magnetite, as indicated by the pulse experiments, and they mediate their input to the brain by the ophthalmic nerve and the trigeminal system. They are not involved in the avian magnetic compass; instead, they
seem to normally convey information on magnetic intensity. Their natural function
appears to be to provide birds with magnetic information as one factor in the multi-factorial navigational ‘map’—not only homing pigeons within their home region, but also migrants when they return to their familiar breeding site or wintering area.
Future histological studies will have to identify their true location and show details of their structure.