A team of scientists believe they can provide the key to an enduring wildlife mystery: how do birds navigate?
Two main theories joust to explain the seemingly miraculous avian compass.
One, supported by research among homing pigeons four years ago, is that birds have tiny particles, called magnetite, in their upper beaks that respond to shifts in Earth’s magnetic field.
Another is that they get a navigational fix from a photochemical compass — a protein that is triggered by light through the bird’s eye and responds to magnetism.
Until now, this latter idea was confined to realm of speculation. No-one had come across a molecule able to respond so sensitively to Earth’s weak geomagnetic force.
Buthave for the first time created a compound that — in a lab but not in nature — confirms that a “chemical compass” of this kind can at least exist. The compound links two biological pigments called carotenoid and porphyrin with fullerene, a spherical carbon molecule.
A burst of light is first used to excite the compound, initiating a transfer of electrons within. This leads to the creation of “radical pairs” of electrons — odd-numbered electrons which team up to form couples, which briefly separate and then recombine.
Electrons have a property called spin. Initially, the spins point in opposite directions, but in this case, a magnetic field causes the spins to become aligned.
In the bird’s built-in compass, when the “radical pairs” recombine, their nicely-aligned spins trigger a biochemical reaction. This is what appears to give the bird information about the magnetic field.
“It’s a proof of concept,” said Peter Hore, who co-led the research with Christiane Timmel at the university’s.
“We’ve demonstrated in principle that an animal could use a chemical reaction to detect not just the presence of the Earth’s magnetic field but also its direction.”
The results chime with the notion that in the bird’s eye is a magnetoreceptor which is triggered by shortwave frequencies, or blue light, that are present in sunlight, he said.
When birds fly at night, they get a navigational “fix” at sunset that may enable them to fly on autopilot until sunrise, Hore suggested.
He theorised that birds could have both types of navigational aid. Magnetite could be used as a “map sense”, enabling the bird to locate itself generally on the Earth’s surface, while a photochemical could provide the “compass sense” for faster and more detailed positioning.
The debate is far from over.
If the photochemical and magnetite ideas are making good headway, it is still unclear exactly how the bird’s brain is able to receive the navigational signals.
And some scientists say that optical cues such as the position of the Sun and the stars and light polarisation, as well as olfactory cues — odours in the atmosphere — could be important signposts.
Another puzzle is how salamanders, frogs and other non-avian animals with in-built compasses are able to orient themselves in relation to the magnetic field. – ya