Light pollution is upending the lives of birds and other organisms that depend on the rhythms of the sun to find food and mates. (Katherine Frey/Washington Post)
It's 2 a.m. You're snoozing in bed. The sun hasn't risen, but the robin has. It is tuk-tuk-tuk-ing at an unnatural hour, waking you up.
If this has happened to you, you're not alone. Around the world, streetlights, store signs and skyscrapers are pouring artificial light into the night, and all that extra illumination is prompting birds to tweet for nearly an extra hour a day on average, according to a sweeping analysis of more than 4 million bird call recordings.
"The day is effectively longer for these birds," said Neil Gilbert, an Oklahoma State University ecologist who co-wrote the study published Thursday in the journal Science.
The finding is another sign of how brightening the night is altering the health of both ecosystems and human beings. Light pollution is upending the lives of birds and other organisms that depend on the rhythms of the sun to find food and mates.
On average, birds prolonged their songs by 50 minutes in the brightest landscapes, chirping earlier in the morning and later into the evening than counterparts in less well-lit locales. Birds with large eyes, open nests and migratory behavior were the most sensitive to light pollution. Among the species most impacted are American robins, northern cardinals, northern mockingbirds, Eurasian blackbirds and killdeers.
For their study, Gilbert and Brent Pease, an ecologist at Southern Illinois University, tapped into a library of bird vocalizations called BirdWeather, a platform that allows both amateur birders and professional ornithologists to place microphones in their backyards and record bird calls.
The pair compared the timing of a whistle, warble or wheep from 583 bird species normally active during the day to the average level of light pollution in its location based on satellite observations. The researchers used an artificial neural network to identify the species making bird calls from March 2023 to March 2024.
"Very few, if any, people who deploy these in their backyard have any idea that people like me and Neil would be using it for some real research," said Pease, who said he has picked up some "really cool warblers" with his own backyard recorder outside Carbondale, Illinois.
"The authors took advantage of an excellent monitoring resource," said Clinton Francis, a sensory ecologist at California Polytechnic State University who also studies the impact of light pollution on birds but was not involved in the study. "The work is especially important because it provides widespread evidence" of changes in bird song around the world.
Kevin Gaston, a University of Exeter ecologist who was also not involved in the research, said most past studies on the biological impacts of light pollution focused on only a few species or a few sites at a time. This new research, he said, is "impressive in the scale of the data used."
The approach, he added, has limitations since satellites only detect light projected upward and computer programs may make errors when identifying bird calls. "But such constraints are almost inevitable in such broad scale studies," he said.
Previous research shows that migratory birds rely on the light in the environment to let them know when and where to go. But the bright lights of cities often draw them off course and cause them to crash into buildings.
Elsewhere in the animal kingdom, light cast by oceanfront buildings cause sea turtle hatchlings to scamper away from the ocean when they would otherwise be heading to sea. In suburbs, streetlamps draw moths away from plants they would otherwise spend the night pollinating. In forests, artificial light puts a damper on the mating pyrotechnics of fireflies.
Humans, too, are being kept up at night. Artificial light can disrupt our sleep cycles and has been linked to higher rates of cancer.
For birds that use calls to find mates, defend territory and warn others of threats, the extra time spent singing during the day may eat into their own rest at night.
But being the early bird can also have its advantages. Past research has shown that some birds exposed to light at night find more mates and feed their chicks for longer.
"Initially, we were tempted to conclude that, 'Oh, it must be bad.' I think we're thinking of our own experiences as humans," Gilbert said. "If we lose an hour of sleep a night, pretty quickly we'll unravel."
"But birds and humans are pretty different," he added.
Unlike climate change, which is politically and technologically difficult to solve, reducing light pollution can be as easy as turning off unnecessary outdoor lighting at night.
"It's just a light switch away," Pease said. "We're optimistic about darkening the night again."