Light pollution is increasing year-round for migratory birds

Many migratory birds in the Western Hemisphere must grapple with increasing rates of light pollution throughout their entire annual migration cycles.

Past research has looked at how city lights at night can interfere with bird migrations as they become disoriented and fly into buildings or towers, or veer off their usual migration route following the lights, consuming critical energy and time they need for their journey.

“It’s been known for over 100 years that migratory birds are attracted to strong sources of light pollution,” said Frank La Sorte, a research associate at the Cornell Lab of Ornithology. “Initially, it was light houses and more recently it’s buildings and communication towers.”

While there’s a wealth of research on how these collisions occur, very little has been conducted on how light pollution rates are changing or how it may impact birds during other stages of their annual lifecycle. La Sorte published one of the first studies on this topic in Environmental Pollution in 2021, looking at how light in urban areas influences bird occurrence throughout the year.

But in a recent study published in Ecosphere, La Sorte and his colleagues tackled the question of how levels of light pollution are changing over time within species’ seasonal distributions.

To conduct the research, the team generated weekly relative abundance estimates for 42 nocturnally migrating species using observations from the eBird citizen-science database.

Then, they intersected these estimates with 22 years of open access light pollution data taken from satellite images.

Overall, the researchers found that light pollution is generally increasing in the Western Hemisphere. It has gone up about 16% in the areas they looked at over the study period. All of the species they examined occured in regions with increasing light pollution levels.

For example, they found bird species in southeastern North America during the breeding season experienced high levels of light pollution at night. In addition, birds in northern South America during the nonbreeding season didn’t have as much light pollution to deal with.

Blackburnian warblers (Setophaga fusca) are also impacted by light pollution in the Western Hemisphere.
Credit: Ian Davies, Cornell Lab of Ornithology

The most dramatic increase in light pollution, the researchers found, was in Central America—an important migration route for many birds. This could create a problem in a bottleneck region of many species’ annual journeys. “They’re forced to encounter higher levels of light pollution than if they had a larger region to migrate across,” La Sorte said.

He added that birds with longer migrations are more at risk of light pollution impacts. “They’re undertaking tremendous, energetically demanding activities,” he said. “Any deviation that requires additional energy and time and increases risk reduces the chances of surviving the journey.”

Some 7% of the area they looked at actually experienced a decline in light pollution. Those small declines, which mostly occur in the northeastern U.S., are likely due to urban decay due to declining human populations and changes in lighting technology, La Sorte said. Birds experienced the least light pollution when they spend the summer breeding there.

La Sorte said bird abundance is skewed toward urban areas with more light pollution during migration. This knowledge has led to “Lights Out” programs in various cities, where people turn out unnecessary lights at night during migratory seasons. “Once the light pollution sources are removed, they can navigate effectively again,” he said.

The authors recommend reversing light pollution trends in southeastern North America during the breeding season and in Central America during migration and the nonbreeding season. In some ways, the United Nations is leading the charge by coming up with global guidelines for light pollution.

“Humans generate air pollution, water pollution—and light pollution,” he said. “Being able to understand what this means for birds and other taxa is important. Fortunately, the adverse effects of light pollution can be easily remedied by turning lights off during peak migration periods.”