How birds use Earth's magnetic field to navigate
Can birds see the Earth's magnetic field? The latest research on navigation suggests they can and even hints that humans may be able to detect it too.
WHY CHICKENS? That was the question. To biologist Dr Ursula Munro, a fellow scientist's suggestion they investigate whether chickens could sense the Earth's magnetic field seemed nonsensical.
"I ridiculed the idea," says Ursula, an ecologist and expert on animal behaviour at the University of Technology, Sydney. "A built-in magnetic compass makes sense in migratory birds, which must find their way across huge distances. But chickens? They don't move far; why would they need it?"
Since the 1960s, scientists have confirmed that more than 20 migratory species of bird use the Earth's magnetic field to help them find their way. In the early 2000s, Ursula herself found that the tiny Tasmanian silvereye navigates magnetically during its annual migrations up the Australian coast.
It seemed that species that took part in long-haul travel were uniquely equipped for this task. Which was why, in 2004, Ursula was dismissive when Dr Raf Freire, then at the University of New England, NSW, suggested testing chickens to see if they too had the knack. Nevertheless, she agreed to collaborate with Raf on a series of experiments using young chickens.
What emerged from that research astonished not only Ursula but also the wider scientific community. "I was gobsmacked," she says.
Early theory of bird navigation
One of the first people to suggest that inanimate objects could exert a force on living creatures was Franz Mesmer, an 18th-century Austrian physician, although his theory of 'animal magnetism' soon fell out of favour.
Then, in the 1950s, scientists noted that caged European robins, which normally migrate southward in autumn, would assemble at the southern end of their cages in the appropriate season.
In the following decade, Wolfgang Wiltschko, a student at Germany's J.W. Goethe University, in Frankfurt - and now the world's leading authority on avian navigation - placed electromagnetic coils around the cages of robins to alter the magnetic field, causing the birds to gather at a different compass point.
This and subsequent investigations by him and his wife, Roswitha, proved beyond doubt that birds can detect magnetic fields and use them to orient themselves.
These days we know that birds are not the only creatures that can do this. Snails, fruit flies, bees, butterflies, salamanders, newts, lobsters, frogs, bats, salmon, trout, whales, sea turtles and the mole rat of East Africa can, too.
While the evidence that these creatures can sense and use the Earth's magnetic field is pretty conclusive, how exactly they do it and what organs they use to do so are the subjects of fiercely competitive research around the world.
How birds use magnetism to navigate
Raf, who now lectures in animal behaviour at Charles Sturt University in Wagga Wagga, NSW, says this makes it an exciting field to be working in. "A lot of important discoveries are happening very quickly," he says. "You can feel the race to try and nail down what the mechanisms are."
Scientists speculate that animals have two distinct magnetic sensing mechanisms in their bodies, each with a different function. In birds, both of these magneto-receptors are believed to be in the head. It's not known if they work simultaneously, independently or jointly.
However, as the Wiltschkos suggested in a 2005 paper, one of the receptors probably acts as a magnetometer - which measures magnetic intensity - and the other as a compass. In animals, this action could be based on two magnetically sensitive forms of iron oxide - magnetite and maghemite. Particles of these minerals in an animal's body align themselves with a magnetic field, affecting cells around them and thus firing off signals to the brain.
Many bird species have tiny bundles of these minerals in the upper beak region, and Raf has found that anaesthetising the nerves in this region impairs a bird's capacity to sense a magnetic field. Different geographic features and zones have differing magnetic intensities.
As a bird flies over them during a migration, its magnetometer may detect these anomalies and enable it to compile a mental map of magnetic signposts for future use.
The second mechanism, the compass, may indicate direction. But unlike a conventional compass, it doesn't distinguish between north and south; it tells the bird only where a pole is and where the equator is.
As Ursula says: "It doesn't matter whether a bird is in the Southern or Northern hemisphere, in autumn it knows it needs to go equator-wards."
How birds us magnetism as a compass
Here's how scientists think the avian compass works. The organs for this mechanism are believed to be in the right eye, but perhaps in the left eye also. Research indicates this magneto-receptor may be based on pigment proteins in the retina known as cryptochromes.
The Earth's magnetic field seems to induce a chemical reaction in these proteins when certain light wavelengths (mostly blue) strike the retina. This results in signals being sent from the eye to the brain via the optic nerve. Some scientists believe this may mean a bird can actually see the magnetic field.
The Earth is one vast magnet, with its magnetic poles situated close to the geographic poles. Magnetic field lines extend away from the Earth at the South Magnetic Pole, travel north and plunge back into the planet at the North Magnetic Pole. So at the poles, the lines appear vertical, at the Equator they appear horizontal, and in-between they align at varying inclinations.
The precise nature of the chemical reaction in cryptochromes is believed to vary according to the angle of the magnetic field lines - their inclination - as they pass through the eye. Inclination, therefore, is a strong pointer to direction. Angled lines may indicate that a bird is close to a pole; horizontal lines may mean the bird is at the Equator.
For the moment, what the magnetic field looks like to a bird is anybody's guess. "The magnetic compass is a side-function of the eye, and magnetic information is primarily mediated by the visual system," Roswitha Wiltschko says. "Yet birds must separate the magnetic from the visual information somehow. How birds perceive this information is impossible to tell."
Illustrators have tried different techniques to depict what they imagine a bird may see. Some have shaded parts of the visual image in grey. Others have varied the colour intensity, with some parts of the image being brighter than others.
Raf, on the other hand, suggests the information might appear on the image as dots or blotches. Overall, the effect might be like the heads-up display projected onto the windscreen of a jet fighter to give the pilot vital information.
There's a theory that - by swinging its head from side to side and thus changing the angle between the magnetic field lines and its eye - a bird generates a moving visual impression of the magnetic field. Some migratory birds scan the horizon in just this way before setting out on a long journey.
They appear to do it more at dusk, when the dim blue-green of the sky may be of the right wavelength for maximum sensitivity to magnetism. In fact, movement, either from head-scanning or from flight itself, may be important for 'seeing' field lines.
If the inclination compass depends on light, logic says it shouldn't work in the dark. Yet some migratory birds seem to be able to orient themselves while flying at night. Ursula attributes this to the fact there is always some light available. "Yes, it's light dependent, but birds can navigate at night using it because there's always light from the Moon and stars. It's never pitch black."
Chickens use magnetism too
Raf is fond of chickens. Much of his work on them has been aimed at improving their welfare. Back in 2004 he was at the University of New England, where he and a team that included the initially sceptical Ursula and the Wiltschkos launched into a series of tests to find out whether the birds were equipped to sense magnetism.
In initial tests, the scientists trained chicks to find a red ping-pong ball hidden behind a small screen.
"We trained them to always search for the ball in a certain direction, say always north, and then we used copper coils to shift the magnetic field by 90° and, hey presto, instead of going north the chickens went east," Raf says. "This meant they were using the Earth's magnetic field to navigate by. It was neat and straightforward."
Subsequent tests proved conclusively that chickens possess not only a magnetic compass, but also an iron-ore-based magneto-receptor in the upper beak area. They were dramatic discoveries.
Although magnetic sensing was known in more recently evolved branches of the bird family, this was the first time it had been seen in an archaic lineage, such as the chicken's. What's more, it had survived thousands of years of domestication.
"Our thinking is that it originated in an avian ancestor, before the chicken line broke off, so it's quite an ancient skill," Raf says.
An implication of this is that all birds, and perhaps most animals, may sense magnetic fields. Raf has no doubt about this. Unlike vision, which requires huge amounts of neural computing power, magnetic sensing is simple and economical. The Earth's magnetic field is omnipresent and receptors can monitor it all the time, providing constant background information.
"Jungle-dwelling chicken ancestors would have used it on their home range - about a kilometre square," Raf says. "It would be hard for them to distinguish trees in the jungle visually, so they would just use something as simple as the magnetic compass to navigate."
Ursula agrees. "Magnetic sensing might simply be one component among the large number of cues available to aid navigation... I believe now that it is everywhere. It makes sense."
If it's common in nature, do humans have it? Some scientists believe so, though they stress it's only a guess. In the 1970s zoologist Dr Robin Baker of the University of Manchester, UK, claimed to have shown that humans could orient themselves magnetically.
However, no other scientist has been able to reproduce his results. In 1992, US scientists published a paper confirming that humans have small amounts of naturally occurring magnetite in their bodies. As for cryptochrome, scientists have known since the 1990s that humans have this pigment protein in the retina. Until recently, though, it seemed only to play a role in setting the biological clock.
Then, in 2011, Professor Steve Reppert, a neuroscientist at the University of Massachusetts, USA, experimented with fruit flies that had been genetically engineered to be cryptochrome-deficient and were poor magnetic navigators.
By splicing the human cryptochrome gene into the flies, he restored their navigational capacity. He'd made a link between human cryptochrome and magnetic sensing.
"Based on our studies, I believe humans have magnetosensing abilities," Steven says. As for how we might perceive and use the Earth's magnetic field, he adds: "We believe that human magnetosensing may aid visual spatial perception rather than acting as a compass for directional information."
So maybe you can't tell north from south on a cloudy day. But if you're blessed with a good sense of direction, perhaps it's all down to your cryptochromes.