Researchers found that exposing great tit nestlings to artificial light at night altered their immune responses, suggesting that light pollution could harm the health of urban birds by potentially making them less able to fend off infections.

Although other researchers have examined baseline immune function, "not so many have looked at the immune response," Ann-Kathrin Ziegler, an evolutionary ecology PhD student at Lund University, in Sweden, told The Academic Times. "The novelty of our study is that we actually look at the immune response."

In the study, published March 26 in the Journal of Experimental Biology, she and her colleagues examined innate immune responses in great tit chicks living in nest boxes in a Swedish forest.

Numerous laboratory studies have found that artificial light at night can take a toll on bird, mammal and invertebrate immune responses, but only a few studies have looked at wild animals, according to Ziegler, who is first author of the study.

"Urban areas are expanding at an increasing rate," she said. "Light pollution at night is one of the main pollutants one can find in urban environments. We know that artificial light at night, or ALAN, as we call it ... suppresses melatonin production, which is a hormone that promotes sleep and regulates circadian rhythms and is also well known to be involved in immune function."

There are two main branches of the immune system in mammals and birds. The innate branch takes care of nonspecific threats. 

"Baseline [innate] immune function is kind of like a fence around your house," Ziegler explained. But when exposed to a potential threat, the innate immune system kicks into gear. "This response is like a police officer or someone that is defending your house." 

The second branch is acquired immunity. Like a force of detectives, each trained to look for a certain type of criminal, acquired immunity is developed after exposure to disease-causing pathogens.

To measure how artificial light at night affects baseline immunity and immune responses after a pathogen challenge, Ziegler and her colleagues began by illuminating 19 nest boxes with an LED bulb at night, simulating urban light pollution. The other 19 received no artificial light. Each nest box had around seven nestlings. After seven nights, the researchers measured immune markers in nestlings' blood samples both before and after an injection of lipopolysaccharide, which mimics a bacterial infection. 

Baseline levels of haptoglobin, a key marker of innate immunity, were similar in both sets of nestlings. But after the immune challenge, haptoglobin levels increased more in dark-night nestlings compared with their counterparts, suggesting that artificial light hampers the birds' ability to mount an immune response. 

Another marker of innate immunity is nitric oxide, a signaling molecule that has been shown to increase after immune challenge in other birds. At baseline levels, nitric oxide levels were higher in dark-night chicks compared with those exposed to artificial light at night, in agreement with another study. 

But, countering the team's expectation, nitric oxide levels decreased after the injection in both sets of birds. Levels declined more steeply in the dark-night birds, which meant the light-exposed birds had higher nitric oxide levels after the immune challenge. 

"It was a bit of a surprise to us that it, overall, declined," Ziegler said. "We took the samples 15 hours after the immune challenge, so it could have been that we just kind of missed the peak." 

One possible explanation for the dark-night birds having lower nitric oxide levels after the injection is that melatonin suppressed the production of this molecule, which has been observed in mouse immune cells, but more research would be needed to evaluate this idea, according to Ziegler.

The researchers suspect that the effects of artificial light at night on nestling innate immunity were regulated by melatonin, a hormone that controls the body's daily processes known as circadian rhythms. Melatonin levels in light-exposed chicks were, on average, 49% lower than in the dark-night birds. In nestlings exposed to artificial light at night, low melatonin levels could affect the immune system directly or indirectly by altering the birds' circadian rhythms, which help regulate immune function.

Light pollution from streetlights, buildings and neon signs is an increasing problem as urban areas continue to sprawl, according to Zielger. This artificial nighttime glow can hamper wildlife reproduction, sleep, movement and foraging and make animals more susceptible to disease, although light pollution could also be beneficial to some birds in certain instances — for example, by helping them adjust to climate change.

While the study's findings suggest that artificial light at night altered great tit immunity, the researchers didn't investigate potential long-term effects on bird health, survival or reproduction. It's also unclear whether artificial light at night also affected the birds' acquired immunity.

Ziegler hopes that improved understanding of the impacts of light pollution on wildlife will ultimately lead to policies that limit urban illumination or consider lighting color, which can ameliorate negative effects on wildlife. She said she would also like to "make people aware that they don't need to light their gardens in the night," something that could also help reduce light pollution.

The study, "Exposure to artificial light at night alters innate immune response in wild great tit nestlings," published March 26 in the Journal of Experimental Biology, was authored by Ann-Kathrin Ziegler, Hannah Watson, Arne Hegemann, Jan-Åke Nilsson and Caroline Isaksson, Lund University; Richard Meitern, University of Tartu; and Virginie Canoine, University of Vienna.