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.
The scant information we do have from ingenious experiments like Emlen’s show just how much animal brains can understand and learn about the natural world.
That information should give us pause as we continue to change our planet. As humans artificially brighten the sky, and as we launch more satellites into orbit that outshine even stars, we may be messing with the cognitive compasses of untold numbers of creatures.
Birds … in a planetarium?
Emlen’s experiments read like something out of a scientifically curious little kid’s dreams. When he was a graduate student at the University of Michigan, Emlen was given the keys to the Longway Planetarium in Flint, Michigan, where he could reign free at night.
“The director closed the planetarium at 10:30, and they gave me the key,” Emlen recalls. “I became nocturnal.” Between experiments conducted there, and later at Cornell University, he pieced together a theory for how the birds navigate.
When Emlen started his work, some things were already known. A husband-and-wife duo from Germany, Edgar Gustav Franz Sauer and Eleonore Sauer, had worked out in the decade prior that migratory birds — which sometimes fly thousands of miles in a single season — look to the stars to get a sense of direction.
The Sauers put birds in outdoor arenas where the only thing they could see was the night sky. And with just the sky as their guide, the birds attempted to fly in their expected migratory direction. They wouldn’t do so on a cloudy night. The Sauers repeated the experiment in a German planetarium, and it worked there, too. Which was amazing: Birds could use information they found in the sky — even man-made replicas of the night sky — to navigate.
But there were still unanswered questions. What were the birds looking at in the night sky, and how were they figuring out the right way?
There were several hypotheses. Some argued that the birds were using an internal clock of sorts to orient themselves to the stars. Stars change their positions over the course of the night, and when viewed from the northern hemisphere, they appear to rotate around Polaris, the static North Star. Perhaps they’re born with an innate sense of time and learn where the stars should be at a given moment. (Similarly, humans know that around sunset, they can find the sun by looking to the west.)
Emlen wasn’t sure that was true. So he decided to find out — with the help of the planetarium, North American indigo buntings, and a special cage he invented with the help of his father (who was also a biologist).
The cage was in the shape of a funnel, and the buntings — a beautiful, sparrow-sized songbird that migrate at night — were placed in the narrow bottom of the funnel. This design, illustrated below, ensured that the birds could only look at what was above them (i.e, the “sky”).
The upper part of these funnels was covered in paper, and the bases of the cages — “just aluminum pudding pans,” Emlen says — featured an ink pad that turned the birds’ feet into stamps. Little avian footprints would appear on whatever side of the funnel the bird attempted to fly toward. The top of the funnel was covered with plexiglass or a wire screen, so the bird wouldn’t get out — hence, no poops in the planetarium.
In the planetarium, Emlen could tinker with the cosmos. He started by setting the stars to a different time of night than it actually was, throwing off the birds’ biological clocks. Yet the birds would still orient themselves in the right direction of their migration. “They were not using a clock,” Emlen says.
So the birds could orient themselves regardless of the time of night. It meant they were focusing on some other aspect of the night sky. But what?
Emlen started on a painstaking process of elimination. As he describes, he “attacked” the expensive planetarium projector, blacking out certain stars systematically. “Let me block the Big Dipper,” he remembers thinking. “Let me block Cassiopeia.” No matter the constellations omitted from the cosmos, the birds could still orient themselves.
“I couldn’t link it to any particular star pattern,” he says. “I had to block out pretty much everything within about 35 degrees of the North Star. And when that happened, the birds acted as though they were clueless.”
The clueless birds were a big clue for Emlen. He knew then that the orientation had something to do with the area around the North Star — but didn’t rely on any of the particular stars around it.
Maybe it was the spot in the sky that doesn’t rotate at all.
A further, ambitious experiment would prove this hypothesis correct. This time, Emlen didn’t just bring birds to a planetarium — he raised some of them inside one. Again, he altered the planetarium projector, not by blocking out stars but by changing the axis of the Earth. He chose a new stationary “North Star” — Betelgeuse — for his chicks to observe.
Remarkably, the birds raised under this altered sky would orient themselves toward Betelgeuse, as it was the fixed point, when they were ready to migrate.
The experiment showed that the birds are primed for nighttime navigation not by an inborn star map, Emlen says, but by paying “close attention to the movement of the sky. They’re hardwired to pay attention to something, which then takes on meaning.”
Emlen is still not sure if the birds look for some sort of constellation to point their way north, once they’ve learned where it is from the motion of the stars. We humans often use the Big Dipper to find north.
“Different birds might use different star configurations,” says Roswitha Wiltschko, a German behavioral ecologist who has conducted similar experiments on bird navigation. “And apparently there is some individual difference in it. This is a part of orientation where we do not know the details yet.”
How many animals look to the stars?
In the decades since these experiments, ornithologists have learned a lot more about how birds navigate. They don’t just use a star compass — they also have a magnetic compass, a sun compass, and even a smell compass. It’s incredibly complex. “All these things intermingle,” Emlen says, and scientists still aren’t sure precisely how these different navigational systems all work together. (They’re especially unsure about how animals use these inputs to inform their mental map of where they are going.)
Scientists don’t have a precise accounting of how many different species of bird navigate by starlight, but experts suspect it is a huge number. More broadly, biologists don’t know how many other species look at starlight. Based on discoveries in the past several years, this ability has already shown up in surprising places.
Consider the dung beetle, which takes its name from its favorite food, namely, um, excrement.
These critters have a very limited visual field, but can actually see the Milky Way in a dark night sky. One particular type of dung beetle lives in South Africa, scavenges for dung, and rolls it into balls away from the source, to protect its food.
This sounds simple. “But for one thing, you have to bear in mind that this ball is usually bigger than the beetle itself,” says James Foster, who studies dung beetles at the Universität Würzburg. “So it’s quite challenging to keep that on course.”
Here’s the amazing part: “They really don’t get lost unless you build them a tiny hat and put that over their head,” Foster says. “They can’t just look around at the ground and work out where they’re going. They really need to be able to see the sky.”
Like Emlen, Foster’s colleagues brought beetles into a planetarium and started switching stars on and off, systematically. They found that on nights where there is a moon, the beetles use it to orient themselves. But if there is no moon, “if you switch off everything else and turn the Milky Way on, then they’re oriented again. So that was what led us to think that they’re using the Milky Way.”
That’s pretty astounding stuff. Starlight from tens of thousands of light-years away, still has enough power to excite the nervous system in the limited eyes of the lowly dung beetle, helping it know where to go.
But this ancient navigation system is also threatened by city lights. “Artificial light … can completely obscure the kind of things that the animals are looking for,” Foster says. “If you put dung beetles on the roof of a building in the middle of Johannesburg, then they become completely lost. It’s just far too bright for them to be able to see the Milky Way, which is the thing they need.”
Foster isn’t sure how many animals on Earth can orient themselves with the stars — no one is — but he suspects it might be more common than currently appreciated. Seals, moths, and of course humans have been shown to use stars. But it stands to reason that changing the night sky — with electric lights and bright, near-Earth satellites that outshine the stars — could continue to mess up the navigation of untold numbers of creatures.