Although it has been known for more than half a century that birds can perceive direction, altitude or location using the magnetic field of the Earth, the precise mechanism that drives this “avian magnetoreception” is poorly understood. A popular theory is the “radical-pair” model, which says that incoming photons excite magnetically sensitive molecules – known as “cryptochromes” – in the birds’ retinas, causing an electron to transfer between two neighbouring molecules, leaving each molecule with an unpaired electron spin.
Depending on the orientation of these molecules to some external magnetic field, the molecule spins either point in the same or opposite directions, so long as the molecules remain excited. This results in the formation of triplet and singlet states, respectively, leading to different neuronal responses in the birds. Because molecules lying along the field lines tend to favour the singlet state, a bird could determine the orientation of the geomagnetic field by comparing the effect of the field on molecules arranged at different angles across the retina. It is also known that cryptochromes absorb light anisotropically – that is, only light of a specific direction and polarization excites the molecules. Also, a certain polarization direction activates a specific subgroup of receptor molecules, and only these go on to form radical pairs and therefore are affected by the magnetic field. All of this suggests that the light-dependent magnetic compass itself is intrinsically sensitive to polarization.
To test the interactions of a bird’s magnetic compass and polarized light, Rachel Muheim and colleagues from Lund University in Sweden studied the behaviour of zebra finches trying to find food inside a simple cross-shaped maze with four arms. The maze was placed on a wooden table within a magnetic coil, which allows the researchers to deflect the horizontal component of the magnetic field.
The birds were first trained to find a food reward – a tray with millet seeds at the end of each arm – after they were released at the centre of the maze. During training, the birds only used the magnetic field to navigate the maze. The set-up also included an overhead light source that would illuminate the maze either with unpolarized light or linearly polarized light. Once the birds had learnt to navigate the maze, the researchers tested the bird’s navigational abilities under different alignments of polarized light and the magnetic field.
Munheim’s team found that the finches were only able to use their magnetic compass when the direction of the polarized light was parallel to the magnetic field. Indeed, the researchers found that the finches were led astray not only when the incident light was completely perpendicularly polarized, but also when only 50% of the light was polarized in that alignment. “We were expecting an effect, but not one so large that it would lead to complete disorientation when the direction of the polarization of light was perpendicular to the direction of the magnetic field,” says Muheim.
Disappearing fields and disarray
The team also found that the birds did not use the polarized light as a separate compass or guide, rather it only affected how the birds perceived the magnetic field. While it is still unclear how the different directions of polarized light in relation to the Earth’s magnetic field affect birds in the wild, the researchers say that the birds use it to accentuate the magnetic field during sunrise and sunset. These are times of day when migratory birds are believed to determine their direction and calibrate their compasses before migrating. Muheim told physicsworld.com that “during sunrise and sunset, when the polarized light in the zenith is roughly aligned parallel to North–South, birds should be able to see the magnetic field quite well, whereas at midday it might ‘disappear’.” In the middle of the day, when the polarized light is approximately perpendicular to the magnetic field, “it can be an advantage that the magnetic field is less visible, so that it does not interfere at a time when visibility is important to locate food and to detect predators,” she adds.
Erik Gauger from Heriot-Watt University in the UK, who was not involved in the current work, says that it is interesting and important because it provides further support for the radical-pair model, which is still a speculative rather than a proven mechanism. He says that while he was not surprised that the performance of the magnetic compass depends on the polarization, he was “very intrigued by the extent to which it does, and that even for only 50% polarized light in the ‘wrong direction’ that is perpendicularly polarized, the ability of the birds to orientate seems to be lost completely”.
However Gauger disagrees with Muheim’s group’s statement that “no viable theory exists on how birds, and most other vertebrates, can perceive polarized light,” citing the phenomenon of Haidinger’s brush, where many humans can see a visual pattern arising from polarized light (see video below). “Learning about Haidinger’s brush, we fully expected the compass molecules to also be sensitive to the polarization of light, but we did not take it further than that. However, it is very nice to see this confirmed,” he says.
Muheim hopes that her team’s finding will allow biophysicists to make more accurate models, and by default include polarization of light as a factor in their models. This should lead to more accurate predictions on where the receptor molecules may be located and how they work.
The work is published in PNAS.