For three years, the experiment wouldn’t work, and Henrik Mouritsen couldn’t figure out why.

He had captured European robins and placed them in funnel-shaped cage in a windowless room. The funnel was lined with blotting paper, which preserved the marks of the robins’ feet as they tried to escape. Typically, the birds would try to flee in a consistent direction. Robins, after all, can sense the Earth’s magnetic field with an internal compass in their heads. Even when they can’t see the sun, moon, stars or any other landmark, this compass helps them find their way.

Scientists first noticed this in the 1950s, and they’ve used the funnel experiment ever since to study the magnetic sense of robins and many