Through a bit of serendipity, researchers at Chicago's Field Museum have shown that the lives of thousands — if not millions — of migratory birds could be saved every year by simply turning out the lights in tall buildings when the workday is over.
The fact that birds fly into lighted windows won't come as a surprise to any bird watcher, but the numbers turned up by the Field researchers are surprisingly high.
During the spring and fall migratory seasons in the years 2000 and 2001, some 1,297 birds died by plunging into lighted windows in just one building along Chicago's lakefront, compared to 192 crashing into the building's unlit windows. That's an overall reduction of 83 percent, according to Doug Stotz, an ornithologist and conservation ecologist at the museum, who has been monitoring bird kills for more than a decade.
"That's an incredible savings from just one building," he says.
Deadly: Bad Weather, Bright Windows
Ornithologists have long believed that lighted buildings wipe out large numbers of migratory birds, but the Field research is the first study to put real numbers on the problem.
The study has its roots in research begun in 1978. At that time Field researchers began keeping track of "bird kills" outside Chicago's huge convention center and exhibit hall, McCormick Place. The building, just about a block from the museum, is in the migratory path of many species, mostly small birds that migrate by night.
In the early years, the lights remained on at all times in the huge building, partly for security reasons, and partly to showcase the dramatic facade.
From late March to the end of May, and from mid-August to Thanksgiving, birds pass over the building without any problem, most of the time. But when the weather turns foul, the birds drop down to a lower altitude, and many of them hit the lighted windows.
"There are periods when we go there for a week or more and there won't be any birds at all," Stotz says. "And then there will be periods where, because of the weather, we will find a hundred or more birds each day."
But until a few years ago, there wasn't any way to measure the true effect of the lights because the lights were always on. Following a major renovation, that practice changed, and the lights were left on only during an exhibit.
Night Flights, Death Tolls
It was at that point that Stotz and his colleagues noticed the dramatic change in the number of bird kills — a difference of 83 percent when the lights were turned off.
Most of the birds are small, including the most common casualty, the song sparrow, and since the birds migrate at night, "people aren't even aware it's happening," Stotz says.
"At nighttime the skies fill up with birds migrating during the spring and fall," he adds. "There are billions of birds that do this, and it's almost all at night for the small birds."
There are a number of theories on why birds crash into windows. Some ornithologists say that if a bird can see through a building, it looks like a pathway through the trees rather than a death trap. Stotz thinks the reason is even simpler. Birds don't know they can't fly through glass, and they are attracted to light, so they unknowingly smash their little bodies into the glass.
McCormick Place may not be typical, or we would likely see a lot more cleanup crews sweeping up hundreds of dead birds from the sidewalks of our cities, but the numbers there are dramatic.