City lights setting traps for migrating birds

Scores of migrating birds are being lured by artificial light pollution into urban areas - which may prove to be deadly ecological traps, a study has found.
Researchers at University of Delaware in the US used 16 weather surveillance radars from US over a seven-year period to map the distributions of migratory birds during their fall stopovers.
Since most of the birds that migrate in the US are nocturnal and leave their stopover sites at night, researchers took snapshots of the birds as they departed.
"Shortly after sunset, at around civil twilight, they all take off in these well-synchronised flights that show up as a sudden bloom of reflectivity on the radar," said Jeff Buler, from University of Delaware.

"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," he said.
The researchers wanted to see what factors shape the birds' distributions and why they occur in certain areas.

"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," he said.

One hazard for birds attracted to city lights is death from flying into high buildings. Buler said that some cities such as Toronto have even gone so far as to institute 'Lights Out' programmes, turning off the lights in tall buildings to deter birds from colliding with them.
Researchers analysed the distributions of the birds in proximity to the brightest areas in the northeast such as Boston, New York, Philadelphia, Baltimore and Washington DC.
"We found an increasing density of birds the closer you get to these cities. The effect goes out about 200 kilometres," Buler said.
"We estimate that these flying birds can see a city on the horizon up to several hundred kilometres away," he said.

Suburban areas, such as people's backyards and city parks harbour some of the highest densities of birds.
When they do get lured into cities, the birds seek out suitable habitat, which can cause concerns from a conservation standpoint as lots of birds pack into a small area with limited resources and higher mortality risks.
"We know there's risk of collision with buildings, collision with vehicles, and getting eaten by cats, which are a major predator," Buler said.
Light pollution created in cities has been increasing in recent years with the advent of LED lights, which are much brighter than the incandescent lights they replaced.