The birds are sensitive even to rapidly fluctuating artificial magnetic fields. These fields had no effect on magnetic materials such as magnetite, indicating that the birds do not rely on simple chunks of magnetic material in their beaks or brains to determine direction, as experts had previously suggested.
The results are reported in the May 13 issue of Nature. The study is the first to reveal the mechanism underlying magnetoreception -- the ability to detect fluctuations in magnetic fields -- in migratory birds.
In the study, Thorsten Ritz, assistant professor of physics and astronomy, and colleagues exposed 12 European robins to artificial, oscillating magnetic fields and monitored the orientation chosen by these birds. The stimuli were specially designed to allow for responses that could differ depending on whether birds used small magnetic particles on their bodies or a magnetically sensitive photochemical reaction to detect the magnetic field.
"We found that the birds faced in the usual direction for their migration when the artificial field was parallel to the Earth's natural magnetic field, but were confused when the artificial field was applied in a different direction," said Ritz, the lead author of the paper. "Since the artificial field's oscillations were too rapid to influence magnetic materials like magnetite, it suggests that the most likely mechanism for magnetic orientation in these birds involves tiny changes to magnetically sensitive chemical reactions, possibly occurring in the eyes of the birds -- we are not sure."
In the experiments, the robins could walk and flutter in their cages but could not fly. The birds oriented well in the Earth's magnetic field alone, but were disoriented in the presence of a broad-band (0.1-10 megahertz) and 7 megahertz oscillating field, aligned at a 24 or 48 degree angle to the Earth's magnetic field. When the same 7 megahertz oscillating field was aligned parallel to the Earth's magnetic field, the robins showed normal migratory orientation again.
"Unlike our senses involving vision, hearing, smell and touch, we do not know what receptors underlie magnetoreception," Ritz said. "Migratory birds have long been known to possess a magnetic compass that helps them find the correct direction during their migratory flights. It has remained unknown, however, how birds can detect the direction of the Earth's magnetic field.
"Now, our study points to what we need to look fora molecular substrate for certain chemical reactions. That is, we can rule out magnetic materials in birds' beaks and elsewhere as being possible candidates. Magnetite in the beaks, however, may play a role in detecting the strength but not the direction of the Earth's magnetic field."
The experiments on the birds were conducted in a six-week period in 2003 in Frankfurt, Germany, in the laboratory of Wolfgang and Roswitha Wiltschko, co-authors of the paper, who developed the behavioral experimental setup used in the study for testing magnetic orientation in birds. During migratory unrest, the birds could move in their cages. Each cage was funnel-shaped, lined with coated paper and measured approximately 1.5 feet in diameter. When the birds moved in the cages, they left scratch marks that were counted subsequently by the researchers and analyzed.
To produce artificial oscillating fields, the researchers fed high-frequency currents from a signal generator into a coil that surrounded four test cages. The coil, with a diameter of approximately two meters, could be moved to change the alignment of the oscillating field. Each bird was tested once a day during dusk for a period of approximately 75 minutes.
Besides the Wiltschkos of J. W. Goethe-Universität, Germany, Ritz was joined in the study by John B. Phillips of Virginia Tech and Peter Thalau of J. W. Goethe-Universität.
The research was funded by the Deutsche Forschungsgemeinschaft and the Fetzer Institute.
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