We've understood that birds can sense the earth's magnetic field for a while now. That's how homing pigeons sense direction, and how lots of other species navigate across incredible distances. The most common explanation was magnetic beaks that act like compasses. The truth of how birds sense direction turns out to be a lot more ridiculously complex, and at the same time, much more user-friendly than a beak compass: they just see the magnetic field. With their eyes.
That is, according to two recently published papers by biologists at the University of Lund in Sweden and Carl von Ossietzky University Oldenburg in Germany. The Swedish researchers studied zebra finches, while the German team studied European robins, and both studies found the same thing. It appears that a protein found in birds' eyes—a protein called a cryptochrome, that's linked to circadian rhythms—lets birds see the earths magnetic field as another layer of their vision, an ability called magnetoreception. The researchers say that more studies are needed to conclusively prove this, this but the evidence is very strong.
The study measured expressions of cryptochromes found in birds' muscles, eyes, and brains. The suspected protein, called Cry4, was shown to be active all the time, not on an oscillating schedule like Cry1 and Cry2, which are cryptochromes responsible for circadian rhythms. According to the studies, this means that it is the best candidate for sensing the ever-present magnetic field. Also, Cry4 is expressed centering around the area of the eyes that gets the most light, which makes sense if it's the basis for vision based magnetoreception. Further, Cry4 is expressed much stronger in migratory birds than it is in chickens, who are usually pretty stationary.
The mechanics of how cryptochromes might translate magnetism into light is equally as strange. Scientists think magnetoreception might be due to something called "quantum coherence." The hypothesis states that when light hits birds' eyes, the cryptochrome proteins produce two free radical electrons which are transferred between molecules on their way to becoming information in the bird's brain. These electrons are linked on a quantum level and are sensitive enough to the ambient magnetic field that it can affect the spin, thus sending information about the earth's magnetic field to the visual processing center of the bird's brain.
So what does Earth's magnetic field look like to a bird? It's impossible to know for sure, but the researchers added a nice graphic that shows an approximation of what they think it would look like. Essentially, it appears like a concave curve of light wrapping around magnetic north and dropping to the east and west like a magnetic rainbow. Looking south, it's the opposite, a convex curve that stretches upwards to the east and west. All in all, it seems like a pretty easy way to navigate. So easy even a pigeon could do it.