Birds and many other creatures are thought to have an internal compass that detects the Earth’s magnetic field. There are several hypotheses about the nature of this compass in birds. One of the recent —— the radical-pair mechanism hypothesis —— has emerged from the field of quantum biology, which investigates physical phenomena at the molecular level within the body. Efforts to shed light on this mechanism are underway. Photoreceptors in the retina of birds have magnetic properties when activated by blue light and respond to the geomagnetic field. Scientists say these signals affect the birds’ vision, enabling them to perceive direction visually.