Monarchs on the move: Why dark skies matter

Few sights in nature rival the breathtaking spectacle of monarch butterflies, their vibrant orange wings fluttering gracefully across continents during their incredible migrations. These majestic insects aren’t just a wonder to behold; they’re crucial pollinators, playing an essential role in maintaining the health of our ecosystems. Yet, these delicate creatures face numerous threats, with one unexpected adversary emerging from the glow of communities: light pollution.

The monarch butterfly’s annual migration is a marvel of nature. Each year, millions of monarchs embark on a journey spanning thousands of miles, traveling from their breeding grounds in Canada and the United States to overwintering sites in central Mexico. This epic migration typically occurs in two phases: southward in the fall and northward in the spring. During these migrations, monarchs rely on environmental cues—including the position of the sun and their internal circadian clocks—to navigate. However, recent research from the University of Cincinnati has revealed that exposure to artificial light at night (ALAN) can severely disrupt this delicate orientation system.

According to the 2022 study, monarchs use a time-compensated sun compass, meaning they interpret the sun’s position in the sky in relation to their internal clock to determine direction. When exposed to ALAN, this internal clock is thrown off balance, and their navigational accuracy collapses. In controlled experiments, monarchs that were exposed to constant light—simulating light pollution—demonstrated disoriented and inconsistent flight behavior compared to those kept under normal light-dark cycles. Even short-term exposure to artificial light was shown to desynchronize their biological rhythms, making it difficult for them to maintain their migratory course. The findings highlight a serious concern: that nighttime lighting, even in rural or suburban environments, could interfere with one of the most iconic and ecologically important migrations in the natural world.

This disruption is particularly alarming as monarch populations are already under pressure from habitat loss, climate change, and pesticide use. The glow of streetlights, billboards, and even our own back porch lights adds yet another challenge to their already perilous journey. But the good news is, this is a threat we can do something about.

The DarkSky community is dedicated to addressing light pollution through the Five Principles for Responsible Outdoor Lighting. Two of these principles—“Useful” and “Controlled”—are especially relevant to protecting monarchs and other migratory species. “Useful” means lighting only what needs to be lit, when it needs to be lit. This can be as simple as turning off an unnecessary porch light during migration seasons. “Controlled” emphasizes the importance of using timers, dimmers, or motion sensors to limit when and how lights operate. By taking steps to limit unnecessary artificial light, we can help monarch butterflies stay oriented and complete their incredible journey.

Light pollution is a human-made problem with a human-powered solution. With a few small changes—flipping a switch, installing a shielded fixture, or choosing warmer-colored bulbs—we can reduce the impact of ALAN on monarchs and countless other species. Let’s preserve dark skies and ensure that the night remains a safe and reliable passage for all creatures that depend on it. Together, we can help monarchs find their way.