For decades, we’ve known that migratory birds use the Earth’s magnetic field to find their way but haven’t found any evidence pointing to how it could possibly work. Considering that these birds navigate using a relatively weak magnetic field* and still manage to cover great distances while flying alone, the whole process seems like magic. And like most things deemed ‘magical,’ this phenomenon remained shrouded in mystery until a theory was put forward. Scientists in the late 1970s noticed that the magnetic orientation of birds was light-dependent. They proposed that a light-sensitive molecule – known as a photoreceptor – inside birds’ eyes may be the magnetic detector they were looking for, and that this molecule is the site of chemical reactions that can be affected by a weak magnetic field.

*The magnetic field on Earth’s surface has an intensity of about 50 microteslas – a fridge magnet has a field of 10,000 microteslas.

Photoreceptor: Structures (i.e. cells or proteins) that are capable of detecting and responding to light.

At first, physical chemists were sceptical of the idea that a chemical reaction could be underlying the birds’ special sense. After all, the energy of interaction of a molecule with Earth’s 50-microtesla magnetic field is around 8 orders of magnitude smaller than the strength of a chemical bond. How could such a weak interaction be responsible for such a precise sense of direction? The answer may come from quantum mechanics and a phenomenon called radical pairs. According to the radical pair hypothesis, light-sensitive molecules create a pair of radicals when they absorb light, containing two unpaired electrons. The radical pair can then appear in one of two states: with both radicals pointing in the same direction or pointing in opposite directions. Applying a magnetic field – even a weak one like the Earth’s – tips the balance between which state is more likely to occur. Scientists now believe that it is the change in the amount of time each state occurs that allows birds to ‘tune in’ to the magnetic field acting upon them.

The radical pair hypothesis is fascinating, but how much evidence for it is there in live animals? Researchers knew of one family of molecules that could use light to generate radical pairs, known as cryptochromes. Cryptochromes have been found in bacteria, plants, fruit flies, and thanks to the work of Henrik Mouritsen and his collaborators from the University of Oldenburg, in the eyes of several bird species. In their 2021 study in Nature, Mouritsen’s team purified cryptochrome from the retinas of the European robin and were able to measure the changing states of the radical pairs as they applied a weak magnetic field to them, just as physical chemists had predicted. Once they had experimental evidence that avian cryptochrome was magnetically sensitive, they purified the molecule from the retinas of other bird species. They discovered that migratory birds like the robin had much more magnetically sensitive cryptochromes than extreme non-migratory birds like chickens and pigeons, supporting the research team’s theory.

Radical: An atom, molecule, or ion with an unpaired electron in its outer shell.

Cryptochrome: A class of proteins found in plants and animals that are sensitive to blue light.

 Journal references

Xu, J. et al. Magnetic sensitivity of cryptochrome 4 from a migratory songbird. Nature, 2021. Vol. 594 (535:541). DOI: https://doi.org/10.1038/s41586-021-03618-9.

Schulten, K., Swenberg, C.E., and Weller, A. A Biomagnetic Sensory Mechanism Based on Magnetic Field Modulated Coherent Electron Spin Motion. Zeitschrift für Physikalische Chemie, 1978. Vol 111 (1-5). DOI: https://doi.org/10.1524/zpch.1978.111.1.001.