Magnetoreception: Migration Secrets Revealed
“Where are those birds going?” might be one of the first questions a child asks. “South for the winter,” someone tells her. Then the question becomes, “Why?” Migration is both one of the most common actions and one of the most mysterious. The reasons for migration seem obvious at first. So many species move to follow their food supply, to avoid the cold of winter, to avoid predators, or to visit safe places to mate. Loggerhead sea turtles cross oceans to nest; salmon migrate from fresh water to the ocean and then back to spawn; monarch butterflies make the trek to overwinter in Mexico; and many bird species (such as the sandhill crane) migrate long distances to and from feeding and breeding grounds.
Finally, if you’ve found the curious sort, they ask, “How do they find their way?” This answer is the hardest to sort out. Instinct is the easy answer, and it seems fairly certain that the directive to migrate, the timing and the maps are somehow genetically encoded. But the exact mechanisms have been puzzling scientists for centuries.
Some of the navigational tools seem obvious (at least until you start considering the butterflies that are hatched along the way). Birds that migrate at night use star patterns; others use landscape features (like mountain ranges, rivers, coastlines) or the placement of the sun; patterns in the winds or water currents can guide others; and still others (such as salmon) use their sense of smell to find their way home. One special trait that many migrating animals seem to have is the ability to sense magnetic fields – to be able to “see” the magnetic poles and use those to orient themselves (called magnetoreception).
Birds may even have two different forms of “magnetic vision.” One is based on having magnetic iron oxide structures in their beaks that appear to help orient them to magnetic fields. In birds, these structures were first discovered in homing pigeons.
Another involves structures called cryptochromes in the right eyes of birds. The theory rests on the cryptochromes creating and the brain perceiving a chemical reaction that is influenced by magnetic fields (electron spin in paired free radicals). The reaction is created using blue light. Studies have shown that birds become disoriented in red or orange light, or if you cover their right eye.
It’s possible that these two magnetic sensors work together – the iron in their beaks seems to measure magnetic intensity, while the cryptochromes seem to be about the direction of the field. The connection with the eye leads researchers to speculate that birds can actually “see” the magnetic field.
Birds are not alone. All the animals mentioned in the first paragraph also are thought to use some sort of magnetoreception in their migration. Monarch butterflies have cryptochromes on their antennae; several kinds of fish have iron oxides in their bodies; and sea turtles have a keen sense of direction that can be disrupted with magnetic fields.
While a magnetic sixth sense seems extraordinary to us, it’s clear that migrating animals use a complex combination of signals to reach their destinations. One study followed night-migrating thrushes. Researchers exposed a group of thrushes to confusing electromagnetic fields, succeeding in causing them to fly in the wrong direction. After seeing the sunset the following day, they reoriented and got right back on track.
A former Nebraskan, Web Editor Jenn Nemec fondly remembers watching the sandhill cranes come and go each year.
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