Secret Senses: Birds "See" Magnetic Field
by Rebecca Hirsch | Today's Science, September 2009
© 2009 Today's Science
Ask any sailor, hiker, or scout: when embarking on a long journey, always bring a compass.
Songbirds know this rule too. Each spring and fall, they migrate thousands of miles, carrying their own internal compasses to orient themselves.
How birds sense Earth's magnetism has long been a mystery. Now a new study suggests that birds may actually "see" the magnetic field. The study shows that a migratory bird's eyes pass information about the magnetic field to the visual center of the brain. The new research, conducted by a team of German scientists, was published in the September 26th, 2007 issue of the online journal PLoS One.
Playing the Field
Songbirds are masters of navigation. Each spring they migrate to the north, where food is plentiful, and each fall they return to the south for the winter. They often fly thousands of miles, usually flying at night under cover of darkness. Some birds are such expert navigators, they will return to the exact tree where they bred the previous year.
How do they do it? This is one of the great scientific mysteries, and naturally it has attracted a good deal of attention. More than three decades ago, German scientist Wolfgang Wiltschko made a breakthrough discovery relating to the question. At that time, people believed that birds relied on the same kinds of visual cues that people use—the stars, for instance. Wiltschko thought that night-flying birds sometimes followed the stars, but he realized that birds also must have a way to navigate on cloudy nights when the stars aren't visible.
Wiltschko's breakthrough experiment came with caged European robins. A caged bird is restless and eager to migrate. It will flap its wings and turn on its perch, facing the direction it wants to fly in. Wiltschko discovered that if he exposed the robin to an artificial magnetic field, one differing in direction from the Earth's field, the bird would turn to face in the direction of the artificial field. Wiltschko realized that birds were able to guide themselves using magnetism.
Hidden Senses
Wiltschko's pioneering work demonstrated that migratory birds were guided by something invisible to us. The discovery, like others made by researchers dealing with many kinds of animals, drove home the fact that animals can sense things that are completely undetectable to us. Perhaps this shouldn't be surprising, but for a long time, people had assumed that what humans can see, hear or otherwise sense was basically the same as what animals can sense.
But this is not the case. Many birds and insects, for instance, see colors outside of our visual range (red to violet). Bees, for instance, can see ultraviolet markings on flowers that make petals look like landing strips. Once the bee has landed, the markings show the insect where to find the nectar and pollen hidden inside the flower.
It is not just colors that are undetectable to us. Elephants and whales communicate using very low frequencies that we cannot hear. Dogs, bats and some fish can hear high-frequency sounds that we cannot hear either. Snakes have heat-seeking systems they use to detect when a tasty animal is near.
Many people have observed peculiar behavior in animals when an earthquake is imminent. It is not that animals are prophetic; rather, they are sensing something very real—vibrations, perhaps—that our bodies simply cannot detect.
A Radical Idea
Wiltschko's findings with caged robins raised a fundamental question: if birds are sensing Earth's magnetism, how do they do it?
Thirty-five years later, scientists are still trying to find the answer. One theory that is gaining ground says that birds might use as a magnetism sensor something called a radical pair molecule. You can understand radical pair molecules if you remember that atoms have pairs of electrons that travel in an orbit around the nucleus. When one electron is stripped away, the unpaired electron left behind makes the atom unstable. That type of atom is called a free radical, and it will react quickly with another radical that is nearby.
In 2000, a team of physicists proposed that a molecule containing a pair of radicals might be a likely magnetic sensor in birds. The reason such a molecule would be a good sensor is that magnetic field influences how easily a pair of radicals will react with each other. In a magnetic field, the radicals don't react as easily as they otherwise would. That tendency of magnetism to shut down reactions between radicals makes a radical pair molecule a likely candidate to be a magnetism detector.
The idea that birds might sense magnetism this way was just that—an idea. Then came the discovery in 2004 that light-sensing molecules called cryptochromes, which can form radical pairs, are found in migratory birds in a most intriguing place: in the retina of the eye. It was also discovered that these particular retinal cells rev up at night as birds are orienting themselves to a magnetic field.
The Eyes Have It
Before the discovery of cryptochromes in bird eyes, many scientists had already begun to suspect the eye might have something to do with magnetic sensing. For one thing, birds turn their head in the direction of a magnetic field, as if they were using their eyes to orient themselves. For another thing, birds can become disoriented when the surrounding light is the wrong color, suggesting that vision and magnetic sensing are somehow connected.
And another piece of the puzzle seemed to fall into place with this finding: scientists had identified a region of the brain, called cluster N, which is active as birds magnetically orient themselves. Put a blindfold on the birds, and the neurons in cluster N become silent. Take the blindfold off, and the neurons light up again. [See A Bird's Eye View of the Earth's Magnetic Field, November 2002].
Tracing the Path
Dominik Heyers and his colleagues at the University of Oldenburg, Germany wanted to see if they could connect the dots to make a complete picture. Heyers and his team experimented with garden warblers, common migratory birds that travel between Europe and Africa. Heyers's team injected the birds with tracers, fluorescent chemicals that travel along nerve fibers and reveal to researchers which parts of the brain are active.
The team injected one tracer into cluster N. (Remember that's the region of the brain that revs up when a birds is magnetically orienting itself.) The researchers injected a different tracer into the bird's eye. After the birds were exposed to a magnetic field, both tracers traveled to the same place, landing in a region of the brain essential for vision. The result suggests that birds are "seeing" the magnetic field.
Bird's-Eye View
Is it really possible to know what an animal is seeing? Heyers believes the birds see the field as an image.
"It's a pity we cannot ask them, but what we imagine is that it is like a shadow or a light spot on the normal vision of the bird," Heyers told Reuters. He admitted that exactly how birds perceive the signal is anyone's guess.
Heyers' result is a contribution but not a complete answer to the question of how birds sense magnetism. Other research has shown that migratory birds also have a magnetic crystal, called magnetite, in their beaks. Magnetite may act like the needle of a miniature compass. Heyers suspects that the two systems—one in the eye, the other in the beak—may complement one another.
It is interesting to note that people also have cryptochromes in their retinas. Did we once have the ability to sense magnetic fields, and lose it when we no longer needed it? Might the ability to sense magnetism still be in our eyes, needing only to be reawakened?
© 2009 Today's Science
Ask any sailor, hiker, or scout: when embarking on a long journey, always bring a compass.
Songbirds know this rule too. Each spring and fall, they migrate thousands of miles, carrying their own internal compasses to orient themselves.
How birds sense Earth's magnetism has long been a mystery. Now a new study suggests that birds may actually "see" the magnetic field. The study shows that a migratory bird's eyes pass information about the magnetic field to the visual center of the brain. The new research, conducted by a team of German scientists, was published in the September 26th, 2007 issue of the online journal PLoS One.
Playing the Field
Songbirds are masters of navigation. Each spring they migrate to the north, where food is plentiful, and each fall they return to the south for the winter. They often fly thousands of miles, usually flying at night under cover of darkness. Some birds are such expert navigators, they will return to the exact tree where they bred the previous year.
How do they do it? This is one of the great scientific mysteries, and naturally it has attracted a good deal of attention. More than three decades ago, German scientist Wolfgang Wiltschko made a breakthrough discovery relating to the question. At that time, people believed that birds relied on the same kinds of visual cues that people use—the stars, for instance. Wiltschko thought that night-flying birds sometimes followed the stars, but he realized that birds also must have a way to navigate on cloudy nights when the stars aren't visible.
Wiltschko's breakthrough experiment came with caged European robins. A caged bird is restless and eager to migrate. It will flap its wings and turn on its perch, facing the direction it wants to fly in. Wiltschko discovered that if he exposed the robin to an artificial magnetic field, one differing in direction from the Earth's field, the bird would turn to face in the direction of the artificial field. Wiltschko realized that birds were able to guide themselves using magnetism.
Hidden Senses
Wiltschko's pioneering work demonstrated that migratory birds were guided by something invisible to us. The discovery, like others made by researchers dealing with many kinds of animals, drove home the fact that animals can sense things that are completely undetectable to us. Perhaps this shouldn't be surprising, but for a long time, people had assumed that what humans can see, hear or otherwise sense was basically the same as what animals can sense.
But this is not the case. Many birds and insects, for instance, see colors outside of our visual range (red to violet). Bees, for instance, can see ultraviolet markings on flowers that make petals look like landing strips. Once the bee has landed, the markings show the insect where to find the nectar and pollen hidden inside the flower.
It is not just colors that are undetectable to us. Elephants and whales communicate using very low frequencies that we cannot hear. Dogs, bats and some fish can hear high-frequency sounds that we cannot hear either. Snakes have heat-seeking systems they use to detect when a tasty animal is near.
Many people have observed peculiar behavior in animals when an earthquake is imminent. It is not that animals are prophetic; rather, they are sensing something very real—vibrations, perhaps—that our bodies simply cannot detect.
A Radical Idea
Wiltschko's findings with caged robins raised a fundamental question: if birds are sensing Earth's magnetism, how do they do it?
Thirty-five years later, scientists are still trying to find the answer. One theory that is gaining ground says that birds might use as a magnetism sensor something called a radical pair molecule. You can understand radical pair molecules if you remember that atoms have pairs of electrons that travel in an orbit around the nucleus. When one electron is stripped away, the unpaired electron left behind makes the atom unstable. That type of atom is called a free radical, and it will react quickly with another radical that is nearby.
In 2000, a team of physicists proposed that a molecule containing a pair of radicals might be a likely magnetic sensor in birds. The reason such a molecule would be a good sensor is that magnetic field influences how easily a pair of radicals will react with each other. In a magnetic field, the radicals don't react as easily as they otherwise would. That tendency of magnetism to shut down reactions between radicals makes a radical pair molecule a likely candidate to be a magnetism detector.
The idea that birds might sense magnetism this way was just that—an idea. Then came the discovery in 2004 that light-sensing molecules called cryptochromes, which can form radical pairs, are found in migratory birds in a most intriguing place: in the retina of the eye. It was also discovered that these particular retinal cells rev up at night as birds are orienting themselves to a magnetic field.
The Eyes Have It
Before the discovery of cryptochromes in bird eyes, many scientists had already begun to suspect the eye might have something to do with magnetic sensing. For one thing, birds turn their head in the direction of a magnetic field, as if they were using their eyes to orient themselves. For another thing, birds can become disoriented when the surrounding light is the wrong color, suggesting that vision and magnetic sensing are somehow connected.
And another piece of the puzzle seemed to fall into place with this finding: scientists had identified a region of the brain, called cluster N, which is active as birds magnetically orient themselves. Put a blindfold on the birds, and the neurons in cluster N become silent. Take the blindfold off, and the neurons light up again. [See A Bird's Eye View of the Earth's Magnetic Field, November 2002].
Tracing the Path
Dominik Heyers and his colleagues at the University of Oldenburg, Germany wanted to see if they could connect the dots to make a complete picture. Heyers and his team experimented with garden warblers, common migratory birds that travel between Europe and Africa. Heyers's team injected the birds with tracers, fluorescent chemicals that travel along nerve fibers and reveal to researchers which parts of the brain are active.
The team injected one tracer into cluster N. (Remember that's the region of the brain that revs up when a birds is magnetically orienting itself.) The researchers injected a different tracer into the bird's eye. After the birds were exposed to a magnetic field, both tracers traveled to the same place, landing in a region of the brain essential for vision. The result suggests that birds are "seeing" the magnetic field.
Bird's-Eye View
Is it really possible to know what an animal is seeing? Heyers believes the birds see the field as an image.
"It's a pity we cannot ask them, but what we imagine is that it is like a shadow or a light spot on the normal vision of the bird," Heyers told Reuters. He admitted that exactly how birds perceive the signal is anyone's guess.
Heyers' result is a contribution but not a complete answer to the question of how birds sense magnetism. Other research has shown that migratory birds also have a magnetic crystal, called magnetite, in their beaks. Magnetite may act like the needle of a miniature compass. Heyers suspects that the two systems—one in the eye, the other in the beak—may complement one another.
It is interesting to note that people also have cryptochromes in their retinas. Did we once have the ability to sense magnetic fields, and lose it when we no longer needed it? Might the ability to sense magnetism still be in our eyes, needing only to be reawakened?