New research suggests that a bird’s eyes contain cells that can detect magnetism as well as light. It is thought that the birds see it in much the same way as they observe trees, rocks and other familiar objects.
One idea is that the field is superimposed on the landscape below — rather like the “head-up” displays found in modern fighter jets.
“We are very excited by this research. The ability to see magnetic fields would solve many questions about bird migration,” said Henrik Mouritsen of the Animal Navigation group at the University of Oldenburg in Germany, which did the research.
At this time of year at least 30m birds are preparing to leave Britain for sunnier climes, many travelling as far as Africa. For decades one of the biggest questions has been how they navigate at night.
Some species fly mainly during the day when there are plenty of obvious navigational clues such as the sun, landmarks and coastlines. However, many small songbirds such as robins, thrushes, flycatchers and warblers migrate mainly during darkness, probably to avoid predators and to keep cool.
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In his research Mouritsen first proved the importance of magnetism by exposing thrushes to a powerful magnetic field at a time when they were migrating northwards. The artificial magnetic field was turned at 90 degrees to the Earth’s normal field and when the thrushes were released they all changed direction to fly west instead of north.
Mouritsen and his colleagues decided to take an educated guess that the perception of magnetic fields lay in the eyes. Next he used garden warblers, which were kept in a cage until their behaviour showed that they were desperate to migrate.
Rather than letting them go, Mouritsen and his colleagues killed them and analysed the cells in the retina at the back of the eyes. In those cells they found proteins called cryptochromes which are highly sensitive to magnetism, undergoing slight structural alterations in response to changes in a magnetic field.
Mouritsen believes that this process, when repeated across the hundreds of thousands of cells lining the retina, allows birds to build up a magnetic map of the ground below. Full details of his research are to be published this week.
His study of the birds suggests that they most likely see magnetic fields as darker and lighter shades or colours which are superimposed on whatever they can see of the landscape.
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However, even if the study has solved one of the mysteries around bird migration, there remain many others. It is, for example, clear that many species have completely different navigational systems.
Sea birds appear to have little ability to detect magnetism. In one of biology’s more bizarre experiments, entitled Waved Albatrosses Can Navigate with Strong Magnets Attached to Their Heads, scientists last year showed that such sea birds were unaffected by magnetism.
When scientists dissected the heads of dead specimens, they found that the most likely mechanism was actually smell, with about 35% of their brains dedicated to the task.
Pigeons, by contrast, do not naturally migrate, having instead a desire to return home and stay there. Their range of powerful navigational tools to achieve this include smell and magnetic detectors in their beaks rather than their eyes.
Oxford University scientists recently showed that pigeons could also memorise man-made landmarks such as roads and railways to find their way home.
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Professor Frank Moore, an expert on bird migration at the University of Southern Mississippi, said migration was deeply risky for any species.
“The mortality is always high and that means there is great evolutionary pressure for migratory species to develop anything that helps them survive,” he said. “A bird that evolves the power to perceive magnetic fields has a big advantage.”