Light pollution makes birds around the world sing longer each day

Cities do not sleep. Streetlights glow, billboards shimmer, and skies rarely turn fully dark. For humans, this makes late-night activity easier. For wildlife, especially birds, this shift in natural light can change the rhythm of life itself.

A new study shows that birds are singing later into the night in areas where artificial light brightens the sky.

Light pollution and bird activity

Brent Pease, assistant professor in Southern Illinois University’s School of Forestry and Horticulture, and Neil Gilbert, assistant professor of biology at Oklahoma State University, gathered global data on more than 180 million bird vocalizations.

The researchers compared these calls with satellite imagery to study how birds respond to artificial night light.

“We were shocked by our findings: Under the brightest night skies, a bird’s day is extended by nearly an hour,” Pease said.

A classroom project that grew

Pease began this project as a creative way to spark interest among undergraduates. He installed a microphone at Touch of Nature Outdoor Education Center, sending live birdsong into the Agriculture Building miles away. Soon, students could watch species activity on building monitors.

To advance the effort, Pease used BirdWeather devices. These tools, linked with Wi-Fi, GPS, and sensors, connect to BirdNET, a system developed by the Cornell Lab of Ornithology and Chemnitz University of Technology. The dashboard revealed in real time which birds were nearby.

How birds act under light

“We started seeing – in real time – which birds were at Touch of Nature, right in our Agriculture Building,” Pease said.

“I realized that I stumbled upon something really important for wildlife research. All of a sudden, we not only know where species are but how they are behaving 24/7.”

Pease explained that BirdWeather allowed researchers to study bird behavior across vast regions and time spans, much like trail cameras transformed mammal research decades ago.

Golden age of bird conservation

“We are just now entering the golden age of avian conservation, all through machine learning and participatory science,” Pease said. He has long worked with citizen-science projects, including sound monitoring during the April 8, 2024 eclipse.

With BirdNET, researchers transformed audio recordings into spectrograms that visually capture each bird’s unique call. These were compared against a database of over 6,000 species.

Please noted that the machine learning algorithm makes it possible to analyze 24/7 audio recordings, which would otherwise take lifetimes to listen to.

“Neil and I are the first, to our knowledge, to apply and analyze the BirdWeather data in this way,” said Pease.

“We have many research options, but first we focused on how birds are responding to global light pollution, which is a growing concern for humans and wildlife alike.”

Big-eyed birds react more to light

The study revealed that the average delay in rest was about one hour, but not all species reacted equally.

“The next question was: why?” Pease said. “What is driving this response by birds? We had the idea that maybe it was a species’ photoreceptor sensitivity – their eyesight. And this turned out to be a key factor.”

“Species with large eyes relative to their body size had a disproportionately stronger response to artificial light at night. They were more sensitive to light at night than species with small eyes.”

Pollution’s effect on birds

The supplementary analysis also looked at noise pollution, which often accompanies artificial light. Birds near roads behaved differently than those farther away, suggesting that noise can shift vocal activity.

However, the researchers concluded that the most consistent behavioral shifts were tied to light, not sound.

Interestingly, nocturnal species showed the opposite pattern. In bright landscapes, they vocalized less and shortened their nightly activity.

For example, in the darkest settings a nocturnal bird might call around 1.6 times per night, while in bright environments it dropped closer to 1.1 times. Moonlight also played a role, amplifying or reducing activity depending on its phase.

Citizen science at its best

Whether these changes benefit or harm birds is not yet clear. Extended time awake might provide more opportunities to feed or attract mates, but it could also mean reduced sleep and higher stress.

“This is citizen-science at its best,” Pease said. “A technology designed for people to check out the species in their own backyards has acquired an unprecedented amount of data since 2021 – over 1.4 billion vocalizations from more than 11,000 locations across the globe.”

“Neil and I are determined to continue to apply the technology for wildlife conservation.”

The study is published in the journal Science.

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