Picture the scene: a moonless April night in the hardwood forests of the southern Appalachian Mountains. Thousands of stars shine overhead. A lone set of wings, iridescent blue in the daytime but rendered colorless in the dark, wends between the trees, ascending and descending with the contours of the land. A common but beautiful species, Passerina cyanea, better known to birdwatchers as the indigo bunting, is in the midst of a 1,200-mile spring migration to its summer range in the Adirondacks. Without fail, in utter darkness, it maintains a steady northward bearing. 

How does P. cyanea do it? By reading the stars. Within the indigo bunting’s brain, no heavier than an olive pit, lies a map of the night sky that allows it, like mariners of old, to maintain a course over hundreds of miles through the night.  

A remarkable study conducted in 1967 by Cornell University ornithologist Stephen Emlen found that indigo buntings could orient themselves based on projected star patterns. Emlen captured birds primed for their fall migration—a behavioral state known as Zugunruhe (German for “migration anxiety”)—and placed them into special enclosures under the artificial sky of a planetarium. Then, Emlen recorded the direction of their hops just before takeoff. Almost without exception, the birds oriented themselves southward, in the direction of their autumnal migration toward Central America, before launching into flight. 

Humans have long used the North Star, or Polaris—the celestial pivot point around which the entire night sky rotates—as a reference point to determine the cardinal directions. Emlen speculated that the birds were doing something similar, reading the sweep of the stars around Polaris to determine their takeoff direction and to maintain a bearing while in flight. (Stars close to Polaris travel in smaller circles relative to stars that are farther away.)

To test his hypothesis, Emlen began systematically wiping away concentric rings of stars around Polaris. For a while, the buntings were unfazed and able to perform their little dance—face south, hop, fly—in spite of the stars that had been removed. But once the erasure of star patterns reached a critical threshold, within 35 degrees of Polaris, chaos ensued. The little buntings, once as unified as a well-practiced dance troupe, lost their sense of choreography and rhythm. They hopped willy-nilly, unable to determine their takeoff direction. Some seemed to lose their Zugunruhe mojo altogether, staring listlessly at the faux night sky. (In the same experiment, Emlen found that he could also reverse the birds’ sense of north and south by projecting Polaris onto the opposite side of the dome.)

Even more fascinating were Emlen’s findings that the birds are not born with star maps in their heads but that they somehow learn the patterns of the cosmos as juveniles. No one is quite sure how they accomplish this, but I like to imagine the young buntings staring skyward from the nest, perhaps trading trilling notes about the proper order of Castor and Pollux, the twin stars of the constellation Gemini, or twittering exuberantly about the vivid orange cast of Arcturus. 

Indigo buntings are not the only animals that use stars as navigational aids. Some species of seals are able to use bright stars, so-called lodestars, to find their way across vast and featureless stretches of ocean. Even the humble African ball-rolling dung beetle looks to the heavens—scouring the night for the diffuse light of the Milky Way—as a directional cue to maneuver its precious excretory prize away from jealous competitors.  

Beyond these and a few other stargazing specialists, dozens more species require darkness for hunting, mating, and avoiding predators. But they have been forced to live upon a planet utterly altered by stray light. Like Emlen in the planetarium, we have erased stars across vast tracts of the sky, throwing animal behavior and migration patterns across the globe into chaos.  

Our love of light and its converse, our hatred of darkness, is having profound effects on us as well, many of which we are just beginning to understand. Studies have shown that overexposure to light during the night has thrown our circadian rhythms out of whack, contributing to sleep disorders along with a cascade of other ailments including depression, diabetes, and obesity. One particularly worrisome study revealed that female nurses who worked night shifts were 32 percent more likely to develop breast cancer than their day-shift-working counterparts. 

The light pollution we are producing is destructive and decadent. It is also a byproduct of ancient anxieties. Reclaiming the night will require a collective mind-shift—a coming to awareness that there is value in darkness. But first we must beat back a more primal aspect of ourselves, the primitive part of our consciousness that still fears dire wolves, saber-toothed cats, and other long-vanished man-eaters, the old ape-brain that seeks safety in the glow of the campfire. Writer Paul Bogard offers a stirring meditation on darkness in his wonderful book The End of Night: “I will readily admit that I am still, especially on windy nights or nights of thunder, afraid of the dark. But I have come to realize that appreciating darkness has little to do with my conquering [of] fear and everything to do with accepting it. . . . It quickens my heart and makes my blood rush around,” he writes. “I don’t want fear so strong that I am incapacitated. But there is fear that comes from being attentive enough that you realize that there is life greater than you, life that was here before you and will be after.”