Seeing magnetic fields
It has long been known that migratory birds can make use of the earth’s magnetic fields to navigate. Birds read the angle that magnetic fields create on the ground and thereby determine how far north or south they are of the magnetic equator and the magnetic pole. But how do they do this? Is there some unknown “magnetic sense”? It seems that birds can actually see magnetic fields-providing the lighting conditions are right. Experiments on redbreasts carried out by a zoo-ecologist at Lund University in Sweden have shown at what light frequencies magnetic force lines appear.
In the early 1990s German scientists demonstrated that in green light birds could find their expected migratory route in relation to the magnetic field. But if the light was yellow or red instead, the birds went astray, taking off in random directions. These German researchers used light-emitting diodes. Rachel Muheim, at the Department of Ecology at Lund, has performed similar experiments but using fiber optics and filters, which yields a narrower and more controlled spectrum.
The classic experimental set-up for such experiments is a funnel-shaped cage where the birds cannot see the sky-birds also use the sun and stars for navigation purposes. Electronic sensors or colors on the walls reveal what route they would like to follow to get out of the cage. Rachel Muheim studied how redbreasts behaved in this situation when they were exposed to green, yellow-green, and red light.
“The birds could navigate in the right direction in low-intensity green light but not in yellow-green light. The frequency difference between green and yellow-green light is very small, which shows how sensitive these perceptions are,” says Rachel Muheim, and continues:
“In low-intensity red light there was a consistent deviation of roughly 60 degrees from the route. We think some of the photo receptors in the eye of the bird are sensitive to magnetism in the green sector of the spectrum. Other photo receptors in the red sector make the bird turn away from the migratory path. When both types of receptors are activated-for instance in yellow-green light-the redbreasts become disoriented.
“Redbreasts migrate during the night. At sunset, when the journey starts, there is low-intensity blue-green light in the parts of the sky that are furthest away from the sun.”
In 1999 Rachel Muheim took part in the Swedish Polar Research Secretariat’s expedition Tundra Northwest in the arctic area of Canada. Her dissertation also presents a study of how two species of North American sparrows behave near the magnetic North Pole. There the magnetic field lines are vertical, and the birds lose their orientation. But when the birds were transported eastward even a tiny degree differential in how the magnet field lines hit the surface of the earth could help the birds navigate. Rachel Muheim and her collaborators found that a differential of as little as 1.3 degrees was sufficient-that’s how sensitive the birds’ magnetic compass is.
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