Animals can navigate by starlight. Here’s how we know.

“No, no, no, no, Brian. No, no, no, no.”

I had asked Stephen Emlen, a Cornell emeritus professor of neurobiology and behavior, what seemed to me an obvious question: When he brought birds into planetariums in the 1960s and 70s, did they ever, um, make a mess in there?

“No poops in the planetarium,” Emlen assures me.

I had called Emlen to talk not about poops, but a series of experiments that have captured my imagination. He brought migratory birds into a planetarium at night and turned the stars on and off, as though erasing them from the universe of a bird’s brain.

Through these experiments, Emlen pieced together what was then a mystery: how birds know which way is which, even flying in the dark of night without the sun for guidance.

We still know incredibly little about animal migration — where they go, why they go, and how they use their brains to get there. Storks migrate from Europe to Africa, and they not only know the route, but can discover locust swarms to feed upon in the desert (long before humans detect the swarm). Whales, in their journeys across the ocean, seem to be influenced by solar storms — but no one knows which part of whale physiology allows them to sense magnetic fields.

How these animals get from point A to point B can be mysterious — and grows even more so as we uncover each new navigational feat. “We just don’t know, really, the fundamentals of animal movement,” science writer Sonia Shah says on the latest episode of Unexplainable, Vox’s podcast about unanswered questions in science.