Researchers at Lund University in Sweden have made a key discovery about the internal magnetic compass of birds. Biologists have identified a single protein without which birds probably would not be able to orient themselves using the Earth's magnetic field.

The receptors that sense the Earth's magnetic field are probably located in the birds' eyes. Now, researchers at Lund University have studied different proteins in the eyes of zebra finches and discovered that one of them differs from the others: only the Cry4 protein maintains a constant level throughout the day and in different lighting conditions.

Cry4 belongs to a group of proteins called cryptochromes. Normally they regulate the biological clock, but have also been considered significant for the magnetic sense. With this study, we now know which of the birds' cryptochromes do what.

"Cry4 is an ideal magnetoreceptor as the level of the protein in the eyes is constant. This is something we expect from a receptor that is used regardless of the time of day," explains Atticus Pinzón-Rodríguez, one of the researchers behind the study.

The conclusion is thus that this specific protein helps the magnetic sense to function, while other cryptochromes, whose levels in the body vary at different times of the day, take care of the biological clock instead.

Last year, Atticus Pinzón-Rodríguez and his colleagues noted that not only migratory birds navigate using a magnetic compass. Even resident birds that do not migrate in the spring and autumn have a magnetic sense and navigate using their internal magnetic compass. He now takes this one step further:

"This and last year's results indicate that other animals, perhaps all of them, have magnetic receptors and can pick up on magnetic fields."

A lot of research remains in order to map in detail how animals discover and use the Earth's magnetic field. What is clear is that it involves chemical reactions that interact with magnetic fields. According to Atticus Pinzón-Rodríguez, this knowledge may be of use when developing new navigation systems.