How do animals migrate without getting lost?
Is it by smell, sight - or via a cosmic 'elastic band'? Sanjida O'Connell investigates
Wednesday 15 March 2006
Harry Marshall, producer of Paranormal Pigeons, due to be screened on Five in May, says pigeons are "the key to unlocking this mystery". They are able to return to their lofts even when they've been released in an unfamiliar location hundreds of miles away, yet scientists have failed to agree on a reason why.
Dr Tim Guilford of Oxford University thinks they do it by using visual landmarks. Guilford, himself a champion paraglider, says: "When you're flying you realise that what is normally a three dimensional landscape becomes very two dimensional, almost map-like.I think that's the way birds see the world."
He and his colleagues have attached GPS devices to pigeons that log their flight. Using Ordnance Survey maps it appears that pigeons do seem to follow major routes, like roads, back home. When they've been released in an unfamiliar location, the pigeons circle, before flying to an obvious landmark like a church and picking up their normal route again.
Even though Guilford's findings back up this suggestion, more than 30 years ago it was shown that pigeons wearing opaque contact lenses could still find their way home. Nor does it explain how birds like the Arctic tern can circumnavigate the globe without ever having been to the place they're headed for.
Professor Wolfgang and Dr Roswitha Wiltschko of Frankfurt University believe it's down to magnetism. They claim that pigeons have a compass in their eye and a magnetometer in their beak to measure the intensity of the earth's magnetic field, and thus whether they are north or south of home. Roswitha says: "You know when you are standing upright and I think it's something similar to that."
The pair could be on to something. In Science, Dr Kenneth Lohmann, from the University of North Carolina, published evidence that suggested baby turtles navigate through the Sargasso Sea using a magnetic map. He exposed loggerhead turtles to a magnetic field generated by an electric coil that mimicked the earth's magnetic field at three key locations along their route. When the turtles were exposed to a field like the one that occurs near Portugal, the turtles paddled south, which is what they would have done had they really been swimming in the area.
A third theory comes from Dr Anna Gagliardo, who believes that pigeons navigate by smell. Gagliardo, from the University of Pisa, claims the birds follow scents blown in on the winds. Harry Marshall agrees. "It's not about following a scent trail," he says. "Each landscape has its own olfactory signature. The birds then remember where these lie in relation to each other, like a patchwork of odours."
Gagliardo has some convincing evidence to back up her claims. She raised two groups of pigeons in sheds made of chicken wire, one of which had glass put round it so that any wind blowing in had to come in through the top. The pigeons in the open roost experienced breezes with distinct regional scents attached; those encased in glass had wind and smells, but did not know which scent had come from which direction. Gagliardo then released the pigeons over a lake so that they would not have any landmarks to use as guides. The pigeons who had been in the glass-enclosed shed were not able to find their way home, but those who had been in the other shed were able to.
But one man disagrees with all these theories. Dr Rupert Sheldrake believes that animals use what he calls "morphic resonance". He thinks that there is a memory in nature, and that objects resonate with it, so that pigeons will return to their loft as if attached by an invisible elastic band that stretches through the cosmos. In a cunning experiment, the Paranormal Pigeon team set out to test his hypothesis. With the help of Ragsy, a pigeon fancier, pigeons were raised in a shed in Sharpness Docks in Gloucestershire. The loft was floated a few miles up and down the canal, showing the pigeons that their home could move - not something that they would ever have come across. Then the loft, minus its inhabitants, was towed out to sea and the pigeons were released from Sharpness. Ragsy waited by the shed, growing increasingly seasick. Eventually the floating shed was towed back. The pigeons were sitting waiting, 55 hours later, when their home reappeared. The cosmic elastic band seemed to have snapped.
Sanjida O'Connell is the author of Sugar: the Grass that Changed the World, published by Virgin Books. Paranormal Pigeons will be shown on Five on 3 May
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