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The early bird gets the worm, or so the well-worn proverb says. But if you’re a bird migrating to your nesting grounds in the spring, arriving too early can be a problem. 

New research suggests that Purple Martins might be mis-timing their spring migration because of artificial lighting that illuminates the sky at night. Each spring, the sky-darting swallows fly thousands of miles from South America’s Amazon Basin to breeding grounds in the eastern United States and Canada. They fly by day, catching insects on the wing as they go, and if they time their departure correctly, they’ll arrive at their breeding site just in time to find a smorgasbord of recently hatched insects. But when scientists analyzed the data from 155 light sensor-wearing Purple Martins between 2008 and 2015, they found that those exposed to bright night light at their wintering grounds left and arrived at their destination eight days early. The scientists speculate that such drastic shifts in migration timing could affect the birds' survival.

How birds and other migratory animals intuit when it’s time to migrate has long been source of human fascination. For Purple Martins and many other birds, the cue triggering that it’s time to go is seasonal changes in light levels. As spring approaches, days get longer. Birds have light-sensitive receptors in their brain and spinal cord that detect these changes in natural light, prompting them to prepare for a long flight.

But how do these light-sensitive receptors, fine-tuned over thousands of years of evolution, respond to the recent phenomenon of artificial light produced by humans? Artificial night lighting can powerfully alter animal behavior, attracting, disorienting, or disrupting a range of organisms from insects and plants to mammals and birds. Sometimes the consequences are fatal: It’s estimated that 1 billion birds die every year after colliding with buildings in the United States, and that their attraction to artificial light is the central cause. Less clear has been whether artificial night at light can affect migration timing.

Kevin Fraser wanted to find out. The University of Manitoba ornithologist has studied Purple Martin migration for a decade by capturing birds at their spring breeding grounds—in Manitoba and Alberta in Canada, and Minnesota, Pennsylvania, and Virginia in the United States—and attaching devices called geolocators, which reveal the itineraries of the birds’ journeys over thousands of miles. Weighing less than a dime, the geolocators are affixed to martins’ backs and recovered when the birds return a year later. Retrieving these sensors from birds that have flown to Brazil and back is “still a thrill every time,” Fraser says, “and the closest we can get to flying with them.”

Researchers use light data from geolocators to calculate birds’ locations, since timing of sunrise and sunset varies geographically. But in this case, Fraser took further advantage of the information. When examining Purple Martin data collected during previous studies, his team noticed spikes of nighttime light much brighter than moonlight, says ornithologist Reyd Smith, one of Fraser’s collaborators on the new study published in . That meant that they could use the same technology that determines the birds’ location to see how much artificial light they were exposed to before migration.

When Smith went back and examined the geolocator data from 155 Purple Martins, she found those exposed to the highest levels of artificial nighttime light—at least 10 nights of it during the 30 days before they departed from Brazil—departed wintering grounds eight days earlier and arrived on their eastern North America breeding grounds eight days earlier, too.

Travis Longcore, a UCLA biologist who studies the wildlife impacts of artificial night light and was not involved in the new study, says the night brightness levels measured by some of the martin geolocators represent “extreme bright light exposure . . . like a dazzling streetlight.” Indeed, Fraser and Smith found that the birds exposed to the brightest lights were in close proximity to cities and towns with an abundance of artificial light.

That such bright artificial lights could affect the birds’ migration is not unexpected, Longcore says. Previous laboratory studies have shown biorhythms affected by light levels much dimmer. “You can entrain caged species with light darker than starlight,” he says—meaning that even tiny amounts of light can modify birds’ internal clocks.

Some experts were more skeptical. Sergio Arturo Cabrera Cruz, migration researcher at the University of Delaware, was surprised by the magnitude of the martin-migration time shift. He would like to see more research confirm that it really was the artificial light that affected the birds’ migration and not other environmental factors like weather, fuel reserves, and other human pressures. The study authors, as well as Longcore, acknowledge that further study is needed. Still, their finding “pushes forward the discussion of light impacts,” Longcore says.

Birds that arrive at their breeding grounds too early can face injury or death due to cold spring weather or lack of food. While this study did not track the martins’ survival, previous research has documented mass mortality events of other swallow species when they arrive at breeding sites during unseasonably cold weather, causing starvation due to reduced insect prey. Whether that’s the case for these early-arriving martins is not yet known.

Although this study focused on one species, the results likely extend to other songbirds. “Purple Martins are not unique in their artificial light exposure, especially at overwintering sites,” Smith says. It could help explain some of the , declines which are more severe than any other North American bird group. “There are so many things going against these birds,” she says. Though light is just one of their many stressors, understanding how artificial night light impacts Purple Martins may help protect them and other birds, especially when solutions are as simple as turning off the lights at night.