How Light Pollution Affects Migratory Birds in the UK

Because migrating birds navigate using the stars in the night sky, bright artificial light can disorient them and throw them off their routes.

Researchers have long examined the dangers of light pollution with migrating birds, but most studies have been done in large North American cities with brightly lit buildings and streetlights. Now, new research has found that migrating birds are disoriented by light pollution in both cities and rural areas in the U.K.

“For nocturnal migratory birds most of the research into the impacts of light pollution comes from North America where the levels of illumination and urbanization are much greater than many parts of the U.K.,” corresponding author Simon Gillings, Ph.D., of The British Trust for Ornithology, tells Treehugger.

“The bird species are also different and it cannot be assumed they will react in the same way," Gillings says. "I wanted to know whether a small U.K. city without brightly lit skyscrapers would have the same impact on migratory birds.”

Gillings chose to study three species of thrushes because they are among the most abundant and vocal migratory birds in the U.K. and would be the easiest to detect in both the city and countryside, Gillings says. 

Recording Night Sounds

For the study, researchers relied on citizen scientist observations. 

“I asked volunteers in the local bird club to host an audio recorder, and had volunteers living along a gradient from dark villages to bright city,” Gillings says. “Each audio recorder was programmed to record nighttime sounds for 2 weeks.”

They had recorders active at 21 locations from late September through late November in 2019. Despite a few early equipment glitches, they ended up with 296 nights that generated 3,432 hours of audio recordings to sift through.

Using computer algorithms, Gillings scanned through all the recordings to find and count the thrush calls amid all the other bird calls and other noises. He was then able to relate the number of calls to the amount of illumination in each volunteer’s backyard.

For all three thrush species studied, call rates were up to five times higher over the brightest urban areas compared with darker villages, suggesting that the birds were likely attracted to the city, Gillings says.

The findings were published in the International Journal of Avian Science.

The Link Between Light and Calling Rate

Researchers aren’t exactly sure why lighting influences the calling rate of birds. 

“This is not absolutely clear—we know birds are attracted to bright lights (e.g. lighthouses) and can get disoriented by very bright light sources. But why they should be channeled over cities is not so clear,” Gillings says.

In the study, the researchers suggest that there could be three plausible explanations to explain the result:

• Birds are attracted to lit areas, so there are more birds present and calling over dark areas.
• Birds fly at lower altitudes over lit areas so more of their calls are detectable.
• Individual birds call more often overlit areas.

The researchers write that radar data analysis seems to support the first explanation, but some results are contradictory.

Whatever the reason that bright lights impact bird calling and migration changes, it’s important to know that it’s happening, Gillings says.

“At a minimum, this suggests that our city lights are changing the flight paths of nocturnal migrants, which might have costs for them in terms of energy expenditure. If they have to stop to refuel they may stop in suboptimal suburban habitats," he says.

"We know from North America that if you build and illuminate tower blocks, migratory birds will collide with these and this can be a major source of mortality," Gillings adds. "So this work shows that European birds behave similarly and that if we build tower blocks we too could see significant mortality for birds that are already at risk owing to other pressures.”