Birds yearn for the bright lights of cities
US radar data reveals migratory flocks head for inner urban roosts, placing themselves in danger. Andrew Masterson reports.
For many species of migratory birds, it seems that the lure of bright lights and big cities might be too difficult to resist - with potentially deadly consequences.
A study led by wildlife ecologist Jeffrey Buler from the University of Delaware in the US, has found that brightly lit urban areas divert nocturnal migratory birds away from country areas full of food and shelter and into suburbs and CBDs where resources are often scarce.
Some birds, too, suffer an even worse fate, flying straight into buildings with often lethal consequences. So prevalent is this behaviour that some cities, notably Toronto, have introduced regulations requiring tall buildings to go dark at night to prevent birds smacking into them.
The reason for the unhealthy attraction to cities, says Buler, is the light itself.
"We think artificial light might be a mechanism of attraction because we know at a very small scale, birds are attracted to light," he says.
"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."
To make their findings, Buler and his colleagues used seven years of data from 16 weather surveillance radars positioned in the north-eastern United States.
The nocturnal flights of flocks of birds show up on the radars as sudden blooms. With this information, the scientists were able to retrospectively determine where each flock had been resting during the day, and at what density.
They found that bird densities increased inside 200 kilometre radii around the largest - and hence brightest - cities in the region: Boston, New York, Philadelphia, Baltimore and Washington, DC.
"We estimate that these flying birds can see a city on the horizon up to several hundred kilometres away," says Buler. "Essentially, there is no place in the north-eastern United States where they can't see the sky glow of a city."
As a result, the researchers found that the greatest densities of bird numbers in the region were not to be found in regional zones, but in city parks and suburban back yards. One suburb, Fairmount Park in Philadelphia, contained more birds than a nearby nature reserve popular with birdwatchers.
The attraction of the bright lights of the city brings with it multiple hazards, Buler says.
"One of the things we point out in this paper is that there might be negative consequences for birds being drawn to urban cities," he explains.
"We know there's risk of collision with buildings, collision with vehicles, and getting eaten by cats, which are a major predator."
And just as young humans from country areas are attracted to the city, despite the hazards, in ever increasing numbers - a recent change in technology seems to be luring more birds into urban ghettoes as each year passes.
"The transition of street lighting from incandescent to LED continues to increase the amount of light pollution," Buler says.
"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. They're still adapting to the light."
The research is published in journal Ecology Letters.