In the bustling metropolitan areas surrounding Washington D.C., artificial light fills the night sky with a constant glow. Now, a study led by researchers at North Carolina State University has uncovered startling evidence of the impact of artificial light on bird populations living in the vicinity of the capital city.
The disorienting effect of light pollution on migratory birds has been well-documented in previous studies, highlighting an increase in collisions with buildings and other deadly hazards. But the new research goes a step further, linking artificial light at night with lower survival rates for two backyard bird species, the gray catbird and house wren, which reside year-round around Washington D.C.
The team analyzed two decades of data, collected between 2000 and 2020 by researchers and citizen scientists through the Smithsonian Migratory Bird Center’s Neighborhood Nestwatch Program.
With the help of volunteers, the researchers captured birds at 242 sites, mostly in backyards of private homes, spanning urban and rural areas of the greater Washington D.C. region. The birds were tagged with uniquely identifiable color bracelets, and then observed by volunteers looking for the color-banded birds in their neighborhoods throughout the year.
Interestingly, the researchers found that survival rates for the American robin increased with artificial light, an anomaly that highlights the complexity of this issue.
Study lead author Lauren Pharr is a graduate student in NC State’s Fisheries, Wildlife and Conservation Biology program.
“These findings are raising awareness about our use of light, and suggest there may be things we can do to help backyard birds that live around us. When it comes to light pollution specifically, there may be things we can do as humans to increase bird survival and help them thrive,” said Pharr.
The study focused on seven species of songbirds, some of the most common in the area, including the American robin, Carolina chickadee, Carolina wren, gray catbird, house wren, northern cardinal, and the song sparrow. “When it comes to urbanization, all of these birds species can persist, so far,” said Pharr.
To delve deeper into the factors affecting bird survival, the researchers combined data from the citizen-science study with maps of light pollution, noise pollution, and paved surface area.
While noise pollution was not found to be a contributing factor, the associations between light pollution and survival were significant for the gray catbird, house wren, and robin.
“This is an important finding; it adds to our understanding that light pollution could have sub-lethal effects on birds,” explained study co-author Professor Caren Coope.
“There is an effort in bird conservation to keep common birds common. We’re lucky we have backyard birds, and we want to keep it that way. If there are things we can understand about the environment that could be affecting their survival, the sooner we can understand that, the better.”
The study also shed light on behavioral changes in birds affected by artificial light. For instance, American robins are known to begin singing earlier in the morning in areas with more light pollution, potentially enhancing their opportunities for mating or foraging.
Additionally, the research pointed to vulnerabilities in some bird species due to migration-related behaviors, even though the gray catbird and house wren do not typically migrate into Central or South America like long-distance migrants.
“There have been other studies that have reported that robins use light to their advantage to forage and find food,” said Pharr. “As far as gray catbirds, some evidence has found they are vulnerable to collisions.”
“There are so many factors that affect a bird’s survival in an urban setting, and they’re all intertwined, affecting predation, physiological harms, and the ability to find prey,” said Cooper.
“Detecting patterns in avian survival rates that vary with artificial light at night is important, and we need more detailed follow-up studies about why that might be happening.”
What makes this study truly remarkable is its use of citizen science, with volunteers and enthusiasts working alongside professional researchers.
“This was my ‘wow’ moment – that we can get all of these people to help us with research and make big impacts,” said Pharr.
“Citizen science is a wonderful and valuable tool, not only to help scientists get data for their projects, and get more eyes, ears and hands on them. The participants also get a chance to understand what we’re doing, why it’s important and to learn alongside scientists.”
The study is published in the journal Urban Ecosystems.