Bird's Eye View....

Author:  Phuongmai Truong
Institution:  UC Berkeley

How do birds know which direction to go during migrating season? While different theories exist to explain this phenomenon, the exact reason had yet to be confirmed. A new study, published in the October issue of Nature, shows that European robins use their eyes to detect magnetic field, as there are magnetic sensing cells in the eyes and a visual center in the brain to process the signal. The author of the study, Henrik Mouritsen from University of Oldenburg, Germany, believes that the result can potentially improve conservation efforts for migratory birds.

The existence of biological magnetic sensors that tell migratory birds which way to fly was widely known among biologists, but the exact location of these natural compasses was unclear. Some biologists believed that iron-based receptors found in the upper beak cells can receive and transmit signal of a magnetic field via a nerve to the bird's brain. Other scientists proposed that the light sensors in the eyes perform the task by sending signals via a different route to a part in the brain called cluster N. In a recent experiment on 36 migratory European robins, Mouritsen and colleagues confirm that the latter hypothesis is true.

It was confirmed that the birds in the experiment were able to respond to both the Earth's magnetic field and laboratory magnets. Afterwards, the researchers performed surgery to deactivate one of the two transmission routes. They either severed the trigeminal nerve, which connects the beak cells to the brain, or damaged the brain cells in cluster N so that light signals from the eye could not be processed. Results showed that the birds with a damaged trigeminal nerve could still orient perfectly, while those with a damaged cluster N were lost against both natural and artificial fields.

Mouritsen says that special proteins in the birds' eyes – called cryptochromes – absorb light and produce highly active molecules with free electrons. Electron spins are sensitive to magnetic fields, so the cryptochromes allow birds to "see" which direction is north. The iron-based cells in the beaks, on the other hand, might play a role in sensing the strength of magnetic fields, Mouritsen suggested.

Despite human attempts to relocate birds from endangered locations, birds tend to fly back to their original migratory locations. With this experimental result and further study on how birds navigate, conservationists may find out a way to guide migratory birds into safe locations. For example, scientists can create artificial magnetic fields to lead birds away from Earth's magnetic field.