Now, through a series of careful behavioral experiments in the laboratory, scientists say they have shown unequivocally for the first time that pigeons have a magnetic sense that goes beyond just a simple magnetic compass. The findings boost the possibility that pigeons -- and maybe other birds as well -- use the Earth's magnetic field to navigate just as sea turtles do.
"Because of their amazing homing ability, pigeons have been used to transport messages since early Egyptian times, and pigeon racing is a popular sport all over the world," said Dr. Cordula V. Mora, a biology fellow in the University of North Carolina at Chapel Hill's College of Arts and Sciences. "In fact, pigeons are often called the poor man's race horse.
"The two main theories are that pigeons smell their way home
or that they have a magnetic map," Mora said. "Our work strongly supports the latter theory, which still remains to be proven in the field."
A report on the studies, which Mora did as part of her doctoral work in New Zealand, appears in the Nov. 25 issue of the journal Nature. Her co-authors are Drs. Michael Walker, Michael Davison and Martin Wild, all professors at the University of Auckland.
"This is a fascinating study in which Cordula managed to train homing pigeons to respond to magnetic fields," said Dr. Kenneth J. Lohmann, Hoggard Distinguished professor of biology at UNC. "It is important news in biology because more than a dozen attempts by others to do this over the years have all failed. Cordula is the first to find a way to make it work."
In the experiments, if the pigeons chose correctly between two platforms located in a tunnel-like chamber, they were rewarded with food, Mora said. Under normal conditions, they would climb onto either of the two platforms randomly in their search for something to eat. But when a magnetic field was present -- induced by coils above and below their chamber -- the birds could be conditioned to climb the correct platform up to 75 percent of the time, which is significantly more often than they would by chance alone.
Once the pigeons were trained to respond to magnetic field stimuli, Mora learned more about the animals' magnetic sense, such as where their likely magnetic receptors are located, Lohmann said.
One set of experiments showed that a small but strong magnet attached to the top of the pigeons' beaks prevented them from going to the correct platform more than half the time. A small, non-magnetic brass weight similarly attached had no such effect, Mora said. Other experiments showed that both local anesthesia in the upper beak area and severing the ophthalmic branch of the trigeminal nerve impaired the birds' ability to detect magnetic fields, while cutting the olfactory nerve, which transfers information about smells, did not.
"Because the magnetic discrimination behavior that the pigeons learned to perform in the laboratory is impaired by various treatments, we can now say that the pigeon's magnetic sense is located in the nasal region and is most likely magnetite-based," Mora said.
Magnetite crystals consist of iron and have magnetic properties. They also have been found in the noses of rainbow trout and have been linked to those fishes' ability to sense magnetic fields, she said.
"For about 30 years, researchers supporting the olfactory hypothesis of pigeon homing have argued that pigeons smell their way home because procedures that impair the sense of smell in the field, like cutting nerves in the nose, disrupt the birds' homing ability," Mora said.
But because those procedures were performed in the nasal region without making sure that the magnetic sense was not simultaneously impaired, the results now have to be regarded with great caution, she said. That is because the disorientated homing behavior could have been a result of disrupting either the olfactory sense or the magnetic sense, or both.
"New experiments need be done in which the sense of smell is impaired without damaging the magnetic sensory system," the scientist said.
At UNC, working with Lohmann, one of the world's top experts on sea turtle navigation, Mora has begun adapting the same procedures to use with the reptiles.
"She has already trained four turtles to press little paddles to get food when a certain kind of magnetic field is present in a tank," Lohmann said. "The experiments are progressing well, and the turtles are learning rapidly. I keep telling Cordula that turtles are smarter than pigeons, but she is not yet convinced."