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> Certain molecules in the  birds' eyes may be the clue:
> 
> https://www.newscientist.com/article/2281998-we-may-finally-know-how-migrating-birds-sense-earths-magnetic-field/
> 
> "We may finally know the secret to how migrating birds can sense Earth’s magnetic fields: a molecule in their eyes called cryptochrome 4 that is sensitive to magnetism, potentially giving the animals an internal compass.
> 
> "The process may result in the animals seeing darker or lighter areas in their vision when they look in the direction of magnetic field lines, says Henrik Mouritsen at the University of Oldenburg in Germany. “You may be able to see where north is as kind of a shading on whatever else you would be seeing.”
> 
> "Previous work has shown that certain species of birds, such as the European robin (Erithacus rubecula), use Earth’s magnetic fields when they migrate, as well as using visual and other cues. Some European robins migrate south every northern hemisphere winter, for instance from Scandinavia to the UK, and return in spring.
> 
> "At least part of this ability is thought to lie in their eyes, because their magnetism sensing is disturbed in the absence of light. Mouritsen has previously shown that when birds are using their internal compass, the information is processed in the same parts of the brain that process vision.
> 
> "Suspicion had fallen on the cryptochrome 4 molecule because it is present in the eye’s light-detecting cells and has a structure that suggests it can be affected by magnetic fields. Now Mouritsen and his colleagues have shown how the molecule reacts to magnetic fields in the lab.
> 
> "The team found that in the presence of light, electrons can jump between different parts of the molecule, and between it and another molecule called flavin adenine dinucleotide (FAD), ultimately leading to the production of a compound called CRY4-FADH*. The process is suppressed by weak magnetic fields."
> 
> Comment: This is a common finding in most migratory species. But a fascinating question for the future is what will happen when the magnetic poles flip as they have done in the past?