Daniel R. Kattnig

In the eye of a migratory bird, a quantum drama quietly unfolds. The protein cryptochrome is its stage. Here, radical pairs are born: two molecules with unpaired electrons whose spins are entangled in a whimsical dance foreseen by John Stewart Bell.

A pigeon flies home from hundreds of kilometers away. Inside its head, in the inner ear, fluid sloshes around, like water in a bottle you shake. When the bird moves, the fluid moves too, helping it maintain balance. Recently, scientists proposed that this same fluid might act as a tiny generator.

Every autumn, a robin weighing as much as a pat of butter lifts off from a branch somewhere near St. Petersburg and flies south toward the Mediterranean, never once checking a map. It is believed that its eye contains a built-in magnetic compass, and the main component of this device is the protein cryptochrome 4a.