From Songbirds to Dung Beetles, These Animals Can Navigate by Starlight

The ability to follow stellar cues is far more common among animals than you might think. But experts warn this could be threatened by light pollution.

Apr 1, 2024 6:00 PM

(Credit: Gergitek Gergi tavan/Shutterstock)

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Astronomy is often called the world’s oldest science, and it’s likely humans have used their knowledge of the night sky to get from point A to point B since prehistoric times. But this ancient art predates us by far longer than that.

Animals, too, follow the stars, and probably have as long as they've existed. In recent decades, researchers have discovered impressive navigation skills in various birds, in seals, even in a few insects — the latter of which have low-resolution compound eyes.

“A starry sky, as beautiful as it looks to us, would look like a Van Gogh painting to them,” says James Foster, who studies dung beetles at the Julius-Maximilians-Universität Würzburg in Germany. “Just completely blurry.”

Yet despite any limitations, creatures of all kinds look to the stars — celestial signposts from trillions of miles away — to move through the world. It’s a behavior scientists have only begun to understand, and one that may be threatened in our hyper-luminous modern era.

Indigo Buntings Use the Night Sky Like a Compass

Bright blue indigo buntings use the stars to navigate at night. (Credit: Weber/Getty Images)

Some of the first evidence that animals follow the stars came from indigo buntings (Passerina cyanea), migratory songbirds with a bright blue hue. In the late 1960s, Stephen T. Emlen, now a behavioral ecologist at Cornell University, placed caged buntings inside a planetarium, where he could switch certain stars on and off to see how the animals reacted.

At first, no matter how he tweaked the indoor sky, the buntings always tried to fly toward the stars in their migration path — based only on these artificial stars, they knew which way was North. But, finally, Emlen unraveled their secret: When he blacked out Polaris (also known as the North Star), as well as everything within a 35-degree radius, the birds became completely disoriented.

In other words, the birds had been homing in on the one point in the sky that never moves, the same guiding light humans have followed for millennia. Emlen gathered that they could identify patterns in the stars rotating around Polaris, using that information to set their route.

What’s more, this ability isn’t innate. For his next act, Emlen raised buntings inside the planetarium, adjusting the sky to rotate around a different star, called Betelgeuse; they treated it just as the wild birds had treated Polaris.

A more recent study in 2021, led by Anna Zolotareva at the Russian Academy of Sciences, found that when the time came for their first migration, pied flycatchers raised without a view of the sky couldn’t pick the right direction. As juveniles they have to study the stars, so to speak, before they can use them as a compass.

(Credit: Gergitek Gergi tavan/Shutterstock)

Birds aren’t fully reliant on the night sky, of course. They also take advantage of the sun’s position, and have an internal magnetic compass that “works around the clock,” as Zolotareva writes.

Why so many navigation options? Because redundancy pays. If you’re visiting a new city and your cell phone dies, Foster notes, “it’s good to have a map in your pocket. You never know when one method might fail you.” For birds, visual signals will obviously be of little help on an overcast day. And solar storms can interfere with the Earth’s magnetic field, rendering that sense useless. In either case, they need some backup options.

Under perfect conditions, when all systems are a go, this blend of internal navigation abilities can also improve directional accuracy. For example, migrating birds often use the sunset — a reliable western bearing — to orient themselves. Then, throughout the night, they recalibrate based on the movement of the stars to stay their initial course.

Harbor Seals Can Learn to Use Telescopes

(Pat Stornebrink/Shuttersock)

In the mid-2000s, a team of German researchers trained two wild harbor seals to peer through a tube (endearingly dubbed the “seal telescope”) and pull their heads back when a star appeared at the far end. It was proof of concept, showing the pinnipeds could indeed make out the dim glow overhead despite their poor vision.

That offered a clue as to how harbor seals navigate what the researchers call a “featureless” ocean environment, especially at night. “Offshore orientation in marine mammals is still a mystery,” the scientists write. So, taking inspiration from the work on migratory birds, they set out to investigate whether seals had similar abilities.

Borrowing from the ornithologists’ playbook, they built a one-of-a-kind floating planetarium. After the seals had spent some time under their makeshift dome, they learned to identify a lodestar — or a star whose path they can use to steer while swimming — with “outstanding directional precision.” Incredibly, this is the same essential technique that Polynesian navigators use to hop between distant islands.

Dung Beetles Can Track the Milky Way

(Credit: Henk Bogaard/Shutterstock)

By comparison with seals and birds, the navigational needs of dung beetles are modest: Once they’ve made their ball o’ dung, they have to roll it away from the competition ASAP. “There’s a big difference,” Foster says, “between traveling 20 meters and traveling across the continent.

Since they’re only going a short distance, there’s no reason to keep long-term tabs on celestial movements. They just keep track of the brightest patch of sky — that is, the Milky Way.

Marie Dacke, who researches animal vision at Lund University, has shown how the beetles use that dense band of light to maintain a straight line as they roll. This ensures they get as far from the dung heap as possible as fast as possible, rather than zigzagging and butting up against rivals or predators.

On overcast nights, they typically just dig down and wait. But if they do get caught out beneath a confusing, starless sky, they’re pitifully helpless. “It’s a bit tragic,” Fosters says. “When we go out to collect them, we have to be faster than the jackals.”

There are thousands of nocturnal animals, many with eyes adapted to the dim world they inhabit. And Foster notes that if they’re sensitive enough to see objects on the ground, they can certainly make the lights above, meaning they have at least the potential for stellar navigation.

“I suspect,” Foster says, “that the number of animals that can orient by starlight is much, much larger than the list we have so far.”

For anyone hoping to grow that list, however, the clock is ticking. As nations around the globe double down on artificial lighting, the night sky is getting 10 percent brighter each year, obscuring the stars for humans and animals alike. In a 2021 study, Foster found that dung beetles at an urban site in Johannesburg were attracted to city lights, leading to more competition between them.

Yet he remains optimistic. Light pollution, unlike other forms of environmental degradation, can be easily reversed — at the speed of light, as he likes to point out. But he also recognizes the trends driving light pollution aren’t promising.

“This is a behavior that we might not be able to observe in most of the developed world for much longer,” he says. “It’s kind of crazy that we don’t even know what we’re losing, or what we’ve lost.”

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