Scientist 1: Migratory birds sense Earth's magnetic field through a light-driven chemical reaction in the eye. Blue light striking a protein called cryptochrome creates a pair of radicals whose chemistry depends on the field's direction. Because this compass needs light, birds should lose their magnetic orientation in complete darkness but keep it under dim blue light.

Scientist 2: Birds detect the field with tiny magnetite crystals in the beak, wired to nerves. These iron-rich particles physically twist to align with the field, mechanically stimulating the nerve regardless of light. On this model, orientation should persist in complete darkness, but severing the beak nerve, or a brief strong magnetic pulse that reorders the crystals, should disrupt it.