Scientists have (again) discovered the secret of pigeons' remarkable ability to navigate perfectly over journeys of several hundred miles. They do it by smell. Research found that pigeons create 'odour' maps of their neighbourhoods and use these to orient themselves. This replaces the idea that they exploited subtle variations in the Earth's magnetic field to navigate. 'This is important because it is the first time that magnetic sensing and smell have been tested side by side,' said Anna Gagliardo, of the University of Pisa, who led the research.

The discovery that birds have an olfactory positioning system is the latest surprising discovery about bird migration. Birds know exactly when to binge on berries or insects to fatten themselves for long flights, and some species recognise constellations, which helps them to fly at night. Birds also travel immense distances: the average Manx shearwater travels five million miles during its life.

Research into navigation has included an experiment in which robins were released with a patch over one eye - some on the right eye, some on the left. The left-eye-patched robins navigated well, but those with right-eye patches got hopelessly lost. 'It is a very strange finding,' said Graham Appleton, of the British Trust for Ornithology . 'It is clear the cues robins use to navigate are only detectable in one eye. Why that should be the case, I have no idea.'

In the Pisa experiments, Gagliardo, working with Martin Wild of the University of Auckland , followed up experiments done in 2004, which showed that pigeons could detect magnetic fields. She argued that this did not mean they actually did.

So in 24 young homing pigeons, horrifically, she cut the nerves that carried olfactory signals to their brains. In another 24 pigeons she cut the trigeminal nerve, which is linked to the part of the brain involved in detecting magnetic fields.

The 48 birds were released 30 miles from their loft. All but one of those deprived of their ability to detect magnetic fields were home within 24 hours, indicating that it was not an ability that helped them to navigate. But those who had been deprived of their sense of smell fluttered all over the skies of northern Italy. Only four made it home. .

Every spring, hundreds of millions of birds head north in order to exploit new resources. Gulls head to the Arctic to make use of the 24 hours of daylight prevailing there, while swallows and other birds leave Africa to exploit the British summertime. The navigation involved in these long journeys is still a cause of considerable debate among scientists. Among the main theories are suggestions that some birds remember visual maps of the terrain they fly over; that they follow the lines of Earth's magnetic field; and that night-time flyers remember star maps of the sky. Of course one has to know where one is to use such navigational aids. There again, young swallows do a solo 5,000 miles migration in the year they were hatched and without a guide to show them the way. How do they know where to head?

However, the discovery of pigeons' prowess at exploiting smells is considered important because their navigational abilities are some of the most acute in the natural world. Pigeons excel at getting home when released in unfamiliar locations. That they achieve such accuracy using smell is all the more surprising. For a substance to be smelled it must be volatile, that is it must enter into a gaseous state at ordinary temperature and be air-borne and carry on air currents to that it can reach the airways, dissolve in mucus and reach the olfactory cells of the receptor. Unfortunately, the wind never blows consistently in the required direction, particularly across continents and seas, and never blows in a straight line but always cyclical. As for “odour maps of their neighbourhoods” – do these extend to a radius of over 800 miles?

So where does this leave us? No matter how many compasses a pigeon has in its pea-sized brain, it will not be able to use these unless it knows where it is currently located, a piece of vital information that pigeon fanciers regularly omit to pass on to the bird when they release it in a strange land maybe 800 miles from home. Professor Tim Guilford of Oxford University made a study and concluded “we simply have no idea”.

A quantum trick might be behind birds' ability to navigate using Earth's magnetic field lines. Some researchers think birds might be able to "see" the magnetic field via photosensitive proteins in their retinas. The theory is that when a photon strikes one of these proteins, it creates a pair of oppositely charged ions, which separate for a fleeting moment before recombining. Each of these ions contains electrons with a quantum property called spin. Initially, these spins point in opposite directions - but in a magnetic field, they tend to become aligned. When the ions recombine, this alignment triggers a specific biochemical reaction, which gives the bird information about the magnetic field.

The idea has a major flaw though. The ions seem to be pulled back together about 10 times faster than researchers think Earth's magnetic field could affect the electrons' spins. Now Iannis Kominis of the University of Crete in Heraklion, Greece, suggests that a known quantum effect might be able to ramp up the impact of the magnetic field in enough time. "Quantum physics comes to the rescue," he says. The "quantum Zeno" effect occurs when repeated measurements of a quantum system are made. While these measurements are taking place particles do not change their state, as if they know they're being watched.

Kominis's calculations show that the force pulling the two ions together might also induce the Zeno effect on the electrons. It would allow the magnetic field to align the spins while the ions are separated by momentarily overcoming the disturbing influences of noise in the biochemical environment, thereby amplifying the magnetic field's influence (www.arxiv.org/0804.2646).

“The Zeno effect might be able to ramp up the impact of the Earth's magnetic field on proteins in birds' eyes” Kominis and his colleagues have already shown that the Zeno effect can increase the sensitivity of a quantum system to a magnetic field. They did this by filling a chamber with a dense gas, thereby building a highly sensitive atomic magnetometer - a device used to detect magnetic fields.

They then applied a magnetic field so weak that many magnetometers would be unable to detect it. But because the gas was so dense, the group showed that the atoms effectively measure each other when they collide and, overall, that keeps the spin of the particles locked in the same state. This made the device strong enough to detect the magnetic field.

Other researchers doubt whether such quantum processes are at play in birds' eyes, however. "I'm a fan of daring hypotheses, but I'm not sure what this theory explains," says biologist Sonke Johnsen of Duke University in Durham, North Carolina. He also points out that the ion reaction theory has bigger problems than the lack of time for the magnetic field to have an effect. "It's not at all clear how to make a directional sensor out of molecules that are freely diffusing and rotating," he says. In other words, the bird might be able detect the field, but not what its orientation is.

Physicist Thorsten Ritz at the University of California, Irvine, says the idea may have merit, however. "It's really cute and worth exploring further," he says, "but I'd want to see experimental tests before I believe it."

All these theories, which fall by the wayside one after another, leaves Science somewhat in disarray. What is it that they are missing? Perhaps Professor Sheldrake may actually have something when he thinks “distant intentionality” (telepathy to those not familiar with the expression), may play a part. Certainly, so far, he has been as right as anyone apart from a Dorset pigeon-racer named Roy C. (I had better not disclose his name on the web)

Roy C says the answer is sex. He says they only race male pigeons and no matter what the weather, sun or no sun, day or night, with or against the prevailing wind his birds invariably make it back to base before the transporter.. He says he segregates the males and females a week before an event then puts them in adjoining pens where they can see one another. The males, maybe hundreds of them, are transported up to a third of the way round the world and released. Sometimes, in the pigeon racing fraternity, this practice is called “widowing” The birds then have but one desire – to get back to the loft and have no idea they are in a race except a personal one for each of them. They are always home, with the females, long before the transporters. With the wind behind them (which effectively negates the smell theory, they may average 100mph.

Maybe Professor Sheldrake and Roy C should get together on this one. They may both be on to something profound.