Science

Light Pollution Is Causing Birds to Nest Earlier—Which Might Not Be a Bad Thing


Birds exposed to artificial lights at night nest up to a month earlier than those dwelling away from humanity’s glow, according to a study published recently in Nature. But, perhaps counterintuitively, this disruption may actually benefit some birds—in part by helping them adjust as global warming alters the rhythms of the natural world.

The new paper offers a continent-wide, multispecies look at the impact of light and noise pollution on birds’ reproductive success, with the hope of giving land managers more concrete information to make conservation decisions. Using data gathered by citizen scientists through the Cornell Lab of Ornithology’s NestWatch program, the study’s authors analyzed more than 58,000 nest observations for 142 species in the contiguous U.S. between 2000 and 2014.

For birds living in temperate climates, lengthening days provide the chief cue that it is time to nest. Street lamps and other artificial lighting may trick avian brains into thinking days are longer than they are, so the researchers were not surprised to see birds nest earlier in areas with light pollution. Because nesting is timed to coincide with peak spring food availability, the scientists had anticipated that light-driven early nesting would disrupt this delicate synchrony and disadvantage the species involved. But “we ended up finding the opposite,” says study co-author Clinton Francis, an ecologist at California Polytechnic State University. “For those nests that were exposed to light, they ended up having higher reproductive success.”

The answer to this puzzle may be linked to another human-caused disturbance: climate change. In a warming world of premature springs, birds must somehow adjust to corresponding shifts in food availability. It is possible the artificial light cues have “allowed these birds to catch up to the effects of climate change, which has caused their resources to essentially emerge earlier in the spring,” Francis says.

There could be other explanations, though, says Jacob Socolar, a postdoctoral researcher at the Norwegian University of Life Sciences, who has studied birds and climate change but was not involved with the new research. “The one that’s most obvious to me is that we know that some species are capable of foraging at night by artificial light,” he says. The illumination could create a kind of 24-hour buffet for birds, better nourishing these species for breeding.

None of these hypotheses mean that light pollution is a good thing overall, though. “Light might be ‘helping’ birds in this one respect,” Francis says, but “we need to look at the bigger pictures of the lives of these animals and the ecological systems they live in.” For example, light pollution has been shown to disrupt hormones and sleep in birds and other wildlife, to disorient migrating birds and to contribute to declines in insects that birds eat.

“Light pollution and noise pollution are these pervasive, previously overlooked facets of global change, whose effects, we are increasingly learning, are big,” Socolar says. Global light emissions are escalating at a rate of roughly 2 percent a year. Even in national parks, the glow of artificial lights has been detected from sources 200 miles away. Similarly, a 2017 paper in Science found that 63 percent of national parklands were exposed to significant human-caused noise.

Noise had a clearly negative impact on birds in the new study, particularly for those living in forests. When exposed to the rumble of airplanes or cars, for example, birds delayed nesting, and their clutch size—the number of eggs in a nest—shrank by 12 percent. One explanation is that noise pollution and the songs and calls of forest-dwelling birds tend to be low-frequency, so resulting interference could disrupt communication that is essential to mating.

With bird populations having declined by 29 percent in North America since 1970, according to a study published last year in Science, understanding how humans impact bird reproduction is crucial to conservation. Most research findings on the effects of noise and light pollution on birds involve behavioral changes, such as how the creatures alter their song in response to noise. These results can be difficult to translate to on-the-ground policy decisions, though, because it is not always clear how a changed behavior may affect a species’ survival prospects. One of the larger goals of the new research, which was partially funded by NASA and affiliated with the National Park Service, is to create a sensitivity index. This index could help park managers understand how new noise or light disturbances affect the breeding success of specific avian species of concern.

Compared with climate change, Francis believes the solutions to excessive noise and light are simpler. Technologies such as lights that turn off when not in use and low-tread, quiet tires are tools that individuals and local governments can employ to hush and darken our environment. “It’s certainly worth people thinking about these small changes that have collectively big impacts,” he says. “They can actually make a big difference for wildlife populations.”


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