Jan. 19 (UPI) -- Humans aren't the only animal drawn to the bright lights of the big city. New research suggests urban light pollution attracts migrating birds, luring them into ecological traps.
As birds in the Northern Hemisphere trek southward during the fall, they make stopovers en route to their wintering grounds. Most species travel from stopover to stopover at night. Scientists were able to use data from 16 weather surveillance radars to track the movements of migrating birds.
"Shortly after sunset, at around civil twilight, they all take off in these well-synchronized flights that show up as a sudden bloom of reflectivity on the radar," Jeff Buler, an ecologist at the University of Delaware, said in a news release. "We take a snapshot of that, which allows us to map out where they were on the ground and at what densities. It basically gives us a picture of their distributions on the ground."
The snapshots allowed Buler and his colleagues to look for factors that could be influencing the distribution of migrating birds among stopover locations. They published the results of their analysis this month in the journal Ecology Letters.
"We think artificial light might be a mechanism of attraction because we know at a very small scale, birds are attracted to light," Buler said. "Much like insects are drawn to a streetlight at night, birds are also drawn to places like lighthouses. Especially when visibility is poor, you can get these big fall-outs at lighthouses and sports complexes. Stadiums will have birds land in the stadium if it's foggy at night and the lights are on."
When scientists looked at bird distribution in relation to major metropolises, including Boston, New York, Philadelphia, Baltimore and Washington, D.C., they found bird densities begin to increase at 200 kilometers, or about 125 miles, from a city.
"We estimate that these flying birds can see a city on the horizon up to several hundred kilometers away," Buler said. "Essentially, there is no place in the northeastern United States where they can't see the sky glow of a city."
The birds' attraction to artificial light increases the odds of collisions with tall buildings. If birds end up in a city, they tend to migrate to what few patches of suitable habitat are available, like parks. With large numbers of birds vying for limited resources, mortality rates can increase. Cities also present new threats, like feral cats.
"Domestic cats could be the largest anthropogenic source of mortality for birds," Buler said. "If birds are being drawn into these heavily developed areas, it may be increasing their risk of mortality from anthropogenic sources and it may also be that the resources in those habitats are going to be depleted much faster because of competition with other birds."
Researchers suggest the problem is getting worse as incandescent lights are replaced by brighter LED lights.
"If you think about it from an evolutionary sense, for all wildlife really, mammals and insects and birds, they've only been exposed to this light pollution for less than 200 years," Buler said. "They're still adapting to the light."