Every year, billions of birds migrate over our heads, many traveling on a long journey between Canada and South America. Yet the question remains: how are they able to navigate so successfully? Some navigation methods include following landmarks (like the Hudson River), using the sun, and possibly using smell. While this feat of navigation is already impressive, it’s now believed that birds may even have a sixth sense that helps with their wayfinding—magnetoreception, which allows them to detect and utilize Earth’s magnetic field.

In 1978, Biophysicist Klaus Schulten proposed a revolutionary idea that magnetically-initiated chemical reactions are the basis of magnetoreception. Like many scientists who were skeptical, you may be thinking that the magnetic field is not nearly powerful enough to affect chemical composition—the amount of energy it takes to break apart bonds must be too great. However, a radical pair actually makes this hypothesis possible. 

Later investigation suggests that a protein called cryptochrome acts as the magnetoreceptor in a bird’s eye. The sun emits all colors of visible light, and when blue light from the sun hits the protein, two molecules with unpaired electrons are formed simultaneously. This is called a radical pair. Electrons have a magnetic field, since they have charge and they spin (either spinning up or down) which produces electric current. The unpaired electrons go into a special “waltz” where they rapidly switch between two spin states, spinning in the opposite direction and spinning in the same direction. The magnetic field can affect which state dominates. Once affected by the magnetic field, the radicals experience chemical change, and signals are sent to the brain. 

Phew. While the concept of magnetoreception may seem complex, it’s worth remembering that even our most basic senses can be difficult to describe to those who have never experienced them. How exactly would you describe sight to a person who has never seen before? It’s hard to put into words what exactly sight is without using a concept that a slightless person would not understand. In the case of birds, their sense of direction is simply a part of their innate ability to navigate the world around them.