Birds May Navigate Using Quantum Effects in Their Eyes

The Earth’s magnetic field at the surface is about 50 microtesla. The energy of interaction between a single molecule and a field that weak is roughly six orders of magnitude smaller than the average thermal energy of the molecule, and the thermal energy is in turn 10 to 100 times smaller than a chemical bond, according to a 2009 paper in PNAS by Peter Hore and Christopher Rodgers. By every standard argument in physical chemistry, a chemical reaction should not be able to detect a 50-microtesla field, let alone use it to navigate.

The leading explanation for how birds do it is called the radical pair mechanism. A light-activated protein in the bird’s retina creates a pair of radicals with entangled electron spins. The magnetic field changes how those spins evolve, and that changes the chemical output. The bird may literally see the field.

The reaction starts with a photon and a protein

The protein at the center of the story is cryptochrome. It sits in the retina of migratory birds, concentrated in the outer segments of double cones and long-wavelength single cones, according to a 2018 study in Current Biology by Christine Nießner and colleagues. Cryptochrome binds a light-absorbing molecule called flavin. When a photon hits the flavin, the flavin absorbs the energy and donates an electron to a nearby amino acid, usually a tryptophan. That electron transfer creates two radicals: a flavin radical and a tryptophan radical. A radical is a molecule with an unpaired electron. The two unpaired electrons, one on each radical, are created at the same instant and their spins are correlated. They form a radical pair.

The pair oscillates between two spin states

The two electron spins in a radical pair can be arranged in two ways. They can point in opposite directions, which is called the singlet state, or they can point in the same direction, which is called the triplet state. The radical pair does not stay in one state. It oscillates between singlet and triplet, driven by the magnetic interactions between the electron spins and the magnetic nuclei in the surrounding molecules. The 2009 PNAS paper calls these interactions hyperfine couplings, and they are what set the oscillation frequency.

If the radical pair recombines to form a stable product while it is in the singlet state, one set of chemical products is produced. If it recombines in the triplet state, a different set is produced. The ratio of the two products depends on how long the pair spends in each state before it reacts.

The magnetic field shifts the oscillation

An external magnetic field adds another term to the spin dynamics. The 2009 PNAS paper explains that the Zeeman interaction between the field and the electron spins can shift the energy levels of the singlet and triplet states. When the field is parallel to the radical pair’s axis, the shift is different from when the field is perpendicular. That means the singlet-triplet oscillation rate depends on the orientation of the radical pair relative to the magnetic field.

If the cryptochrome proteins in the retina are fixed in a consistent orientation, and if the radical pairs form in those proteins at a consistent angle, then the chemical product ratio becomes a function of the bird’s orientation in the Earth’s field. The bird does not need to measure the field’s strength. It needs to measure the field’s direction, which is encoded in the product ratio.

A 2016 paper in PNAS by Thomas Player and Peter Hore described a sharp feature in the spin dynamics that they called the quantum needle. When the spin coherence persists for longer than a few microseconds, the sensor output contains a spike that could deliver heading precision sufficient to explain the navigational behavior of migratory birds in the wild.

Cryptochrome 4 is the leading candidate

Not all cryptochromes are the same. Birds have several cryptochrome genes, including Cry1a, Cry1b, Cry2, and Cry4. The 2018 Current Biology study found that Cry4 is upregulated during the migratory season in night-migratory European robins, but not in domestic chickens. Cry4 protein is localized in the outer segments of double cones, which is exactly where a light-dependent magnetoreceptor would need to be.

A 2021 paper in Nature reported that the photochemistry of Cry4 from the European robin is magnetically sensitive in vitro, and more so than Cry4 from non-migratory species. That was the first direct evidence that a protein involved in vision has the magnetic sensitivity required for a compass. The QuantumBirds project, funded by the European Research Council and reported on CORDIS in 2025, confirmed that Cry4 from the robin shows many of the properties required of a magnetic compass sensor. Spectroscopic studies showed that the radical pair lifetime and the magnetic field effect are in the right range.

The coherence has to last long enough

Quantum coherence is fragile. The radical pair exists in a superposition of singlet and triplet states, and that superposition has to survive long enough for the magnetic field to influence it. In a warm, wet biological cell, thermal noise and molecular collisions usually destroy coherence in femtoseconds. A 2011 paper in Physical Review Letters by Erik Gauger and colleagues calculated that superposition and entanglement are sustained in the avian compass for at least tens of microseconds, which the authors described as extraordinarily long compared to any artificial molecular system. That calculation is a theoretical result based on the radical pair model. It has not been measured directly in a living bird.

The QuantumBirds project reported on CORDIS in 2025 that spin dynamics simulations have shown that the operation of a radical pair magnetoreceptor cannot satisfactorily be described without an exact quantum mechanical treatment. The project concluded that avian magnetoreception may be a genuine example of quantum biology.

The evidence is not yet conclusive

The radical pair mechanism is still a hypothesis. A 2017 paper in Biophysical Journal by Peter Hore and colleagues at the University of Oxford used computational modeling to test whether the current form of the mechanism can explain the behavioral effects of radiofrequency fields on bird orientation. Experiments have shown that weak oscillating magnetic fields at certain frequencies disrupt the magnetic compass of migratory birds. The Oxford team found that even with generous assumptions about the radical pair properties, the predicted effects of radiofrequency fields are too small to explain the observed behavioral disruption. The lead author said the current understanding of the model cannot explain any of the reported behavioral results.

The same paper raised the possibility that the mechanism might be missing something, or that the radiofrequency fields might be affecting a different behavior entirely, such as motivation. The QuantumBirds project is attempting an isotopic swap test that could provide a definitive answer. The plan is to substitute carbon-13 for carbon-12 in the flavin molecules of living birds and test whether their orientation changes under radiofrequency fields. If it does, the radical pair mechanism is confirmed. The project’s 2025 report on CORDIS describes the pipeline as created and the behavioral tests as underway.

A 2026 paper from the Okinawa Institute of Science and Technology, reported by EurekAlert, developed a quantum optimal control algorithm to describe how a magnetic field can steer a quantum system from one state to another. The authors hope the mathematical framework will help bring quantum biology experiments out of the theoretical realm and into the lab. They note that despite the hypothesis being proposed nearly 50 years ago, there is still no conclusive experimental evidence for or against it.

What the 50 microtesla field can do

The field is weak. That has not changed. But the radical pair mechanism does not rely on the field doing chemical work. It relies on the field shifting a delicate quantum oscillation that is already underway. The 2011 coherence calculation and the 2016 quantum needle paper suggest that the spin system can be sensitive enough to detect the field if the coherence lasts long enough. Whether the coherence actually lasts that long in a living bird, and whether the radical pair model in its current form can account for the radiofrequency disruption experiments, are the questions the next round of experiments will answer.