Light Pollution Is Causing Birds to Nest Earlier, Mitigating Some Effects of Climate Change

But two wrongs don’t make a right, as both problems are altering the birds’ biology

A yellow and gray songbird is perched on a branch holding long, stringy nesting material in its beak. The background is mottled green and brown.
Longer days signal to birds when they should breed and lay their new clutch of eggs, and they match up their timing so that their chicks are born when the springtime's bounty is at its peak. Nathan Rupert via Flickr under CC BY-NC-ND 2.0

People might admire the glowing, golden aura of a city illuminated against the night sky, but the consequences for wildlife aren't so beautiful. Light pollution from artificial sources at night disrupts how animals process their environment, and it's triggered crashes in insect populations, an uptick of migratory birds colliding into buildings and even changes in underwater ecosystems.

In a new discovery, scientists found that birds living in areas with high light pollution are nesting around a month earlier than usual, reports Drew Higgins for Scientific American. This shift could spell bad news since chicks would hatch before springtime's peak, leaving the parents without abundant resources to care for their hatchlings. But in a surprising twist, the researchers found that birds exposed to light had better reproductive success, the team reported last month in Nature.

After months of short days and long nights, spring starts to emerge as daylight hours grow longer. The lengthening days signal to birds when they should breed and lay their new clutch of eggs, and they match up their timing so that their chicks are born when the springtime's bounty is at its peak. But as a result of warming temperatures, spring is springing earlier than usual. Now, if birds don't also breed early, they'll be late to the game, leaving them with fewer resources and a lower chance of their chicks' survival, Maya L. Kapoor reported for High Country News in 2017.

Scientists have noticed that some birds are moving their breeding time up a little earlier to adjust to the effects of a warming climate, Sarah Kennedy reported for Yale Climate Connections in 2018. This new study suggests that light pollution could actually give birds another signal to start breeding earlier, which could be helping them survive.

The team collected data on more than 58,000 nests from 142 different species across North America by looking at a giant data set compiled by citizen scientists through NestWatch, a program that monitors the bird reproduction. The data revealed that in open environments, birds are nesting up to a month earlier than usual. And in forested areas where light is reduced, it's up to 18 days earlier, according to a press release.

Study author Clint Francis, an ecologist at California Polytechnic State University, tells Scientific American that cues from light pollution have "allowed these birds to catch up to the effects of climate change." Since birds depend on changes in daylight to start breeding, artificial lighting may make them think the days are longer, so they start earlier.

Jacob Socolar, an ecologist researcher at the Norwegian University of Life Sciences, offers an alternative theory. He tells Scientific American that some birds are known to forage at night under artificial lighting, so they could be working overtime to prepare themselves for breeding. Either way, this isn't necessarily good news.

"Light might be 'helping' birds in this one respect," Francis tells Scientific American. "[But] we need to look at the bigger pictures of the lives of these animals and the ecological systems they live in."

Birds depend on their internal clocks to tell them when to breed, lay eggs, forage for food and migrate. Light pollution can throw off that delicate system, which causes changes in their behavior, bodily functions and growth, Jane Kay reported for Environmental Health News in 2014.

All of these are critical factors to consider, especially given that North America's bird population has dropped by 29 percent since the 1970s, according to a study published in Science last year. Conservationists are now scrambling to safeguard the birds' futures by understanding the various origins of this dramatic decline, and light pollution is part of that.

"Land managers and conservation practitioners, for example, can prioritize the habitats and species at most risk from light and noise pollution and better assess the environmental impacts of new developments, as well as mitigate existing ones," co-author Neil Carter, a conservation ecologist at the University of Michigan, says in the press release. "We also hope our results can motivate individuals and communities to reduce their own light and sound footprints."

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