How songbirds could read the Earth’s magnetic lines
Every year, small migratory songbirds weighing less than 50 grams fly thousands of kilometres alone at night navigating to their seasonal wintering grounds with centimetre precision. Navigating by the stars, landmarks and smell, they can also sense the direction of Earth’s weak magnetic field. Despite over 50 years of research, the underlying biophysical mechanism of this remarkable magnetic sense remains obscure. The ERC-funded QuantumBirds(opens in new window) project reveals that the answer appears to lie in quantum physics. Bringing together quantum mechanics, spin chemistry, behavioural biology and biochemistry, researchers tested a radical hypothesis: that these birds ‘see’ Earth’s magnetic field through transient, light-induced chemical intermediates called radical pairs, located in cryptochrome proteins in their retinae.
How birds navigate using quantum mechanics
“At first glance, the notion that a chemical reaction could serve as a compass seems preposterous,” notes joint principal investigator Peter Hore. Individual molecules are so feebly magnetic that their interaction with Earth’s magnetic field is a million times weaker than a standard chemical bond. However, this thermodynamic argument overlooks the unique quantum mechanics of short-lived radical pairs. “When light hits these proteins, the generated radical pairs exist in a quantum superposition state, undergoing coherent oscillations known as quantum beats. Given this delicate quantum state, the yield of the chemical reaction is highly sensitive to the direction of an external magnetic field. This effectively encodes navigation data directly into the bird’s visual system,” explains Hore.
What is cryptochrome-4a in birds’ eyes?
To test this, QuantumBirds tackled three questions. First, can avian proteins actually act as magnetic receptors? The team proved they can, demonstrating that cryptochrome-4a(opens in new window) from the European robin is sensitive to weak magnetic fields – the first direct evidence of a visual molecule possessing these precise properties. To achieve this, they built highly sensitive spectrometers and a high-throughput facility that purified over 40 protein variants across different species. The team went on to explore whether retinal neurons actually encode this cryptochrome-derived magnetic information. By adapting neuroscience techniques typically used on rodents, the team developed the world’s first non-magnetic two-photon calcium-imaging instrument for recording magnetic signals in avian retinal nerve cells. Surprisingly, they discovered that the inner retina operates under constant oxygen shortage. This explains decades of past experimental difficulties and opens the door to imaging magnetically responsive cells. Ultimately, QuantumBirds investigated whether cryptochrome is the primary compass molecule. “We are on the brink of a definitive answer through conducting a sophisticated isotopic swap. Computer simulations indicated that substituting natural carbon-12 with carbon-13 isotopes in the eye’s flavin molecules would change how birds respond to disruptive radiofrequency fields,” outlines co-principal investigator Henrik Mouritsen. “Having created a pipeline to introduce these carbon-13 flavins into living birds, behavioural tests are underway to establish whether their orientation changes. A successful result will provide undeniable proof of this quantum compass,” he added.
Future technology inspired by avian magnetic sensing
Understanding avian navigation mechanics could allow conservationists to ‘trick’ endangered migratory species into accepting climate-resilient habitats as their new homes, overcoming their natural instinct to return to degraded sites. Remarkably, this same avian compass is providing a blueprint for technology. Because the identical quantum physics is found in organic LEDs, scientists could replicate these principles to build cheap, non-toxic magnetic sensors. Parallel research into the birds’ cryptochrome could enable creating alternative navigation systems for environments where GPS is blocked or compromised.
