The mystery behind how birds navigate might finally be solved: it’s not the iron in their beaks providing a magnetic compass, but a newly discovered protein in their eyes that lets them “see” Earth’s magnetic fields.
These findings come courtesy of two new papers – one studying robins, the other zebra finches.
The fancy eye protein is called Cry4, and it’s part of a class of proteins called cryptochromes – photoreceptors sensitive to blue light, found in both plants and animals. These proteins play a role in regulating circadian rhythms.
There’s also been evidence in recent years that, in birds, the cryptochromes in their eyes are responsible for their ability to orient themselves by detecting magnetic fields, a sense called magnetoreception.
We know that birds can only sense magnetic fields if certain wavelengths of light are available – specifically, studies have shown that avian magnetoreception seems dependent on blue light.
This seems to confirm that the mechanism is a visual one, based in the cryptochromes, which may be able to detect the fields because of quantum coherence.
To find more clues on these cryptochromes, two teams of biologists set to work. Researchers from Lund University in Sweden studied zebra finches, and researchers from the Carl von Ossietzky University Oldenburg in Germany studied European robins.
The Lund team measured gene expression of three cryptochromes, Cry1, Cry2 and Cry4, in the brains, muscles and eyes of zebra finches. Their hypothesis was that the cryptochromes associated with magnetoreception should maintain constant reception over the circadian day.
They found that, as expected for circadian clock genes, Cry1 and Cry2 fluctuated daily – but Cry4 expressed at constant levels, making it the most likely candidate for magnetoreception.
This finding was supported by the robin study, which found the same thing.
“We also found that Cry1a, Cry1b, and Cry2 mRNA display robust circadian oscillation patterns, whereas Cry4 shows only a weak circadian oscillation,” the researchers wrote.