Humans perceive the world around them with five senses—vision, hearing, taste, smell and touch. Many other animals are also able to sense the Earth's magnetic field. For some time, a collaboration of biologists, chemists and physicists centered at the Universities of Oldenburg (Germany) and Oxford (UK) have been gathering evidence suggesting that the magnetic sense of migratory birds such as European robins is based on a specific light-sensitive protein in the eye. In the current edition of the journal Nature, this team demonstrate that the protein cryptochrome 4, found in birds' retinas, is sensitive to magnetic fields and could well be the long-sought magnetic sensor.