Navigating With A Built-In Compass
Researchers are finding surprises in the ways animals navigate using the Earth's magnetic fields
You're young and inexperienced, but it's time to fend for yourself. Before winter comes, you must find the way from your Canadian birthplace to a warm haven in Argentina.
That's the challenge that faces young bobolinks in their first autumn. "Bobolinks have the longest migratory path of any songbird in the New World," points out ornithologist Robert Beason of the State University of New York at Geneseo, who studies the ways bobolinks steer a course using the Earth's magnetic field. "I figured if any bird is really good at navigating, it has to be these guys." Little did he imagine when he started the research 15 years ago that his subjects actually might be able to see magnetic fields--about as mind-boggling as finding that they have X-ray vision.
Scientists used to think that migratory birds relied only on cues that humans also could see, such as landmarks and star formations. When ornithologist Wolfgang Wiltschko, now a professor at the University of Frankfurt in Germany, was starting his research career in the 1960s, a colleague had just found that migratory birds kept in a planetarium would change direction when the stars were shifted. "But we knew that a lot of bird migration takes place on starless, cloudy nights," says Wiltschko. "We were interested in finding out what other cues the birds would use."
When Wiltschko exposed European robins to an altered magnetic field in a laboratory, they reoriented themselves to the artificial field. His discovery--which took place more than 30 years ago--opened up new lines of research into the surprising ways that birds sense and interpret magnetic information.
Like any good pilot, the bobolink carries a compass. Recent research suggests that cells in the bird's head contain magnetite, an iron oxide crystal that aligns with magnetic north like a tiny compass needle. Scientists think these cells may serve as receptors that send directional information to the brain. Many other animals apparently also have such cells: Magnetite has been found in the heads of migratory fish, sea turtles and humpback whales. Of all the wildlife navigators, birds so far are the best studied.
To confirm that magnetite functions as part of the bird's compass, Beason has captured migrating bobolinks and tinkered with their sense of direction by treating the birds with a strong magnetic pulse that reverses the polarity of the magnetite in their bodies. (His study subjects are released unharmed in the spring.) Before the remagnetizing treatment, bobolinks kept in circular cages hop toward the southeast, which is their normal migratory direction in the fall. After the treatment, they change direction and hop north, just as you would expect if their compass sense depends on magnetite.
But when Beason recently used Novocain to numb the nerve that scientists think connects the brain with the magnetite cells, the birds went back to their southeast orientation. That means they must have a second way of sensing magnetic fields, one that still works when input from the magnetite cells is switched off.
One of the senses that is still switched on is vision, and Beason and other researchers have been studying how birds respond to different wavelengths of light. Beason has found that when bobolinks are exposed only to red light, they become disoriented. In green, blue or white light, however, their sense of direction remains intact. The theory is that light-sensitive pigments in the birds' eyes serve as magnetic sensors. When green, blue or white light strikes these pigments, their electrons become energized, and the pigment molecules behave like weak magnets. The visual information is then relayed from the bird's eyes to its brain. When deprived of those wavelengths, the bird loses its sense of direction.
No human can know what this ability is like. But Beason suggests a way to imagine the birds' experience. "Arbitrarily, let's say the blue cones in the eye are sensitive to magnetic fields," he says. If you were a bobolink looking toward north or south, that part of your visual field would be intensely blue. "When you looked away from the poles--east or west--there would be absolutely no blue."
Scientists think the pigment mechanism is similar to a simple compass that indicates magnetic-field direction. The magnetite receptor, on the other hand, may be part of a much more sophisticated system that allows an animal to pinpoint its position by taking into account subtle variations in the Earth's magnetic field. "To do this requires a much greater sensitivity to the Earth's magnetic field than using the field simply as a compass," says ornithologist Kenneth Able of the State University of New York at Albany.
This sensitivity could explain some otherwise mysterious feats of animal navigation. How else could newts and homing pigeons find their way home after being taken to a strange, distant location, or green turtles find their nesting beach on tiny Ascension Island, a dot in the open ocean midway between Brazil and Africa?
The geomagnetic field is generated by molten iron moving inside the Earth's core. Imagine a huge bar magnet embedded in the center of the globe, aligned north to south. Lines of magnetic force wrap around and through the Earth, running between the magnetic poles like the lines between segments of an orange. In simplified terms, the field lines sprout out of the magnetic South Pole at a 90-degree angle and then curve back toward the planet to circle it and get drawn into the magnetic North Pole, again at a 90-degree angle.
When the lines reach the equator, they are horizontal to the Earth's surface. Birds sense the angle of these field lines to the Earth's surface at different latitudes--called inclination angle--and use it to navigate. "They interpret inclination angles to tell which direction is poleward and which is equatorward," says Wiltschko.
The first studies of birds using inclination to navigate were done on species that breed in Europe and North America. Wiltschko and his colleagues later found that two birds restricted to the Southern Hemisphere, the silvereye and the yellow-faced honeyeater of Australia, also use inclination to steer a course during migration. A long-distance migrant like the bobolink, which crosses the equator twice on its yearly migrations, must reverse the heading on its inclination compass during the trip. The bird starts out each migration, whether from the Northern Hemisphere or Southern Hemisphere, by heading toward the equator. After it crosses the equator, the bobolink's inclination compass switches to a poleward heading.
Loggerhead turtles also use an inclination compass. After young loggerheads hatch on the Florida coast, they swim into the open Atlantic. The hatchlings often will travel as far east as waters off the shores of Africa before returning to their natal beaches as adults. But if they wander too far north or south, they never make it back to start a new generation. That's because loggerheads can't survive the cold temperatures outside the warm water currents bounded by the southeastern United States, eastern Central America and the western coast of Africa.
Marine biologists Kenneth and Catherine Lohmann of the University of North Carolina have found that hatchling loggerheads use an inclination compass as a guide to keep them in safe waters. In 1993, the Lohmanns used artificial magnetic fields to test the behavior of young loggerheads in their laboratory. Hatchlings exposed to an inclination angle found on the northern boundary of their usual range swam south-southwest; those exposed to an inclination angle found near the southern boundary of their range swam northeast.
Wiltschko and other researchers once thought of birds' magnetic sense as a backup, used only when clouds obscure the stars. But they have found that the magnetic compass is crucial--and it's apparently genetically encoded. A fledgling raised indoors with no exposure to the sun, stars or other landmarks still orients in the right direction in the fall, using cues from the geomagnetic field.
In experiments on garden warblers that migrate from Europe to Africa, Wiltschko and two colleagues found that the birds cannot steer by stars alone. The warblers' inborn directions tell them to head southwest in the Northern Hemisphere's autumn. But young warblers raised in a laboratory free of all magnetic cues got it wrong, heading due south even though they had artificial stars to guide them. It turns out that while the birds can use stars to find which way is south, they also need their magnetic compass to find the correct deviation to the west.
Magnetism may be even more crucial for marine animals that rarely see the stars. Fin whales seek out areas of low magnetic-field intensity during the fall and winter, evidence they may use magnetism to find their way. Errors in magnetic navigation may help explain why whales sometimes strand themselves onshore; stranded pilot whales, for example, are found most often where areas of low magnetic-field intensity intersect with coastlines.
Like the old-time mariners who consulted the stars as well as their compasses, birds combine information from the stars and the magnetic field. Beason has found that bobolinks under an artificial night sky will change direction if the magnetic field is altered, but they don't all switch at the same time. About half the birds change course the first night, and the rest follow over the next three nights.
That's apparently because not all the birds consult the magnetic field every night. If their cages are covered so that they can't see the sky at all, the birds all change direction on the first night. "It's like looking at a compass and picking out a tall tree or a mountain," says Beason. "You walk until you get there and don't check your compass again until you've reached that landmark."
Cross-referencing between the stars and the magnetic field is important because the two guideposts can give different directions. The stars always indicate true north and south, the endpoints of the Earth's rotational axis.
The magnetic poles, however, differ from true north and south, and they even change position over time. Magnetic north is now in Canada, almost 1,000 miles from the geographical North Pole. The angle between the true and magnetic north--and between the true and magnetic south--is called declination, and it varies depending on where you are on the planet. It is small near the equator and increases as you travel north or south.
Human navigators were trying to adjust for the gap between the geographical and magnetic poles as early as 1538, when Portuguese naval commander Joao de Castro began charting worldwide variations in declination. Of course, birds like the Savannah sparrow already had the problem worked out. At the high latitudes of the sparrows' breeding grounds, the angle between true and magnetic north is large. As the birds migrate south, the angle goes through a big change, growing much smaller. Ornithologists Kenneth and Mary Able, both of the State University of New York at Albany, have found that Savannah sparrows use celestial cues to make adjustments to their magnetic compass and compensate for the changes in declination.
Each species seems to have evolved a navigational system to suit its own life-style. Savannah sparrows breed in the Far North but only migrate as far south as the Gulf of Mexico. Anywhere they travel, and whatever the local declination, the stars always provide a reliable reference point. Savannah sparrows use the stars to recalibrate their magnetic compass many times throughout their lives.
The bobolink doesn't experience the large degrees of declination that confront breeding Savannah sparrows. But since the bobolink migrates into the Southern Hemisphere, it loses sight of the northern stars and must learn to cope with new celestial patterns as it travels. So it makes sense for bobolinks primarily to make their way using magnetic cues.
Come fall, these master navigators will head south again, a nation of tiny pilots that will pass over us by night, keeping their course in ways we're still struggling to understand.
California writer Sharon Levy learned to love her compass after getting profoundly lost on a backwoods birding trip.