Birds are being lured to their death by artificial lights in big cities, a new study has shown.
Between 4 and 5 October, nearly 1,000 birds were killed when they collided with an illuminated glass building in Chicago.
Although fatalities of this magnitude are rare, scientists have warned that light pollution poses a serious and growing threat to migrating birds – with artificial lights drawing them into what can be “an ecological trap”.
In the largest study of its kind, published in the journal Nature Communications, researchers used weather radar data to map bird stopover density in the United States.
They discovered that artificial city light is a top indicator of where birds will land while migrating.
Cities however are a “less-than-ideal” migration rest spot for birds, the study warned, as buildings that lead to collisions, scarcer food, less habitat, and more people and cats all pose a risk to life.
Lead author Kyle Horton, an assistant professor at Colorado State University’s Department of Fish, Wildlife, and Conservation Biology, explained why birds’ migration stopover places are so important.
He said: “Migration is a risky and exhausting time in a bird’s life. Birds migrate hundreds to thousands of miles – sometimes burning half their body mass along the way.
“Finding a good place to rest and refuel is critical for migrating birds to survive and thrive once they reach their location.
“If you’re on a cross-country trip and there’s no fueling stations, then you’re stranded. If birds don’t have a good spot to rebuild energy supplies, migration can’t happen.”
The casualties between 4 and 5 October at Chicago’s McCormick Place Convention Centre were mostly songbirds – a species that benefits humans by pollinating plants, distributing seeds, and eating insects that plague gardens.
Though the collisions at the convention center were rare in their scale, studies suggest that mass fatalities involving 100 or more birds are all too common.
It is estimated that nearly one billion birds collide with buildings in the US every year.
However, knowing birds’ migration stopover patterns “can help in the development of conservation plans”, the study says, with Professor Horton and his research colleagues now working with non-profit and government organizations to encourage people to turn off lights.
He commented: “Lighting in cities involves a lot of stakeholders, making it a complicated issue.
“There can also be social pressure to leave lights on, with some people finding them aesthetically pleasing.
“But light pollution harms people too. It can disrupt humans’ circadian rhythms, leading to health problems including depression, insomnia, cardiovascular disease, and cancer.
“We don’t often think about light as a pollutant, but it checks all the boxes of what pollution is.”
Something that can help tackle the issue of light-related bird fatalities is a tool called BirdCast, which provides bird migration forecasts and real-time maps from weather radar.
The collaborative project, created by CSU, the Cornell Lab of Ornithology, and the University of Massachusetts, creates alerts that notify people of when birds are flocking near their city – pinpointing which nights are most important for reducing light pollution.
Other actions that can be taken include retrofitting windows with gridded dots or lines that highlight the building to birds, or lowering the brightness and softening the color of lights.
White or blue lights are the worst for wildlife, while warmer hues, such as red, orange, and yellow, are less attractive.
This was proved in 2016 when, based on conservation research, the Federal Aviation Administration started requiring communication towers to use flashing red lights – dramatically reducing bird collisions in “a literal blink”.
Professor Horton added: “Public awareness of bird migration habits would also be a good place to start to help protect them from light pollution.
“Most people might not realize that birds migrate at night.”
The study, which provides the first continent-wide maps of migration stopover hotspots in the US, paired 10 million radar observations with landscape and other place-based information to try to explain why birds choose to rest where they do.
It found that the top predictor for stopover density was elevation, which provides context for where birds are flying but does not explain why they are flying there.
This was followed by light pollution, which represents the highest human influence on bird migration.
Geoff Henebry, co-author and professor at Michigan State University, commented: “Our study is notable in that it combines big data from multiple spaceborne sensors, and a lot of processing from the weather surveillance radar network, to address key questions regarding the influence of urban areas on bird migration.”
For the study’s authors, the solution to the problem they discovered is simple – at least when it comes to birds.
Professor Horton said: “If we turned off all lights tonight, there would be no birds colliding because of lights tonight. The impact is immediate and positive for birds.”
Produced in association with SWNS Talker