Birds versus windows

Huge numbers of birds die every year in the United States after flying directly into the windows of buildings. That number falls somewhere between 365 million and 988 million, a new estimate finds. Between two and 10 out of every 100 U.S. birds may die this way.

The birds may not see the windows. Or they may mistake a reflection for the real sky. Either way, an average of 24 birds die every year at any given skyscraper, the new study found. By comparison, a single small building may kill, on average, around two birds each year. But comparing only one skyscraper with one small building can give the wrong impression about which buildings might be most dangerous for birds, notes Scott Loss. He’s an ecologist at Oklahoma State University in Stillwater who worked on the new study.

His team found skyscrapers are responsible for less than 1 percent of bird deaths resulting from window collisions. Homes and other buildings that are up to three stories tall account for a far larger share: 44 percent. The largest share of these deaths, 56 percent, occurred when birds smashed into the windows of buildings four to 11 stories tall.

The United States has more than 6,500 of these lower-rise buildings for every skyscraper. And that leads the researchers to conclude that windows in nonskyscrapers pose the biggest risk to birds.

“It’s death by a million nicks,” Scott Loss told Science News. His team’s findings appear in the February issue of Condor: Ornithological Applications.

Some types of birds are more likely to die from colliding with windows, the scientists found. Birds migrating north from Central or South America are especially vulnerable. Loss and his coworkers suspect that these birds, some of which travel by night, may get confused when they see a building’s artificial lights.

The new estimate suggests windows still lag behind cats as a cause of death. Research by others indicates cats may kill up to three times as many U.S. birds each year as windows do.

Windows and cats are just the two biggest threats of immediate death that stem from human activity. (People may also reduce bird populations in other ways, for example by building on the woodlands, sandy beaches and other habitats the birds call home.) Additional birds die after smashing into windshields or the blades of wind turbines. However, Loss says that from what he knows, none of those other direct risks “come close” to those posed by building windows.

Loss also concedes that scientists have a hard time coming up with an accurate death count for birds. No system exists for counting each and every bird that dies by window-crashing. So to estimate the total risk posed by window glass, he and his coworkers combined the results from 23 studies of bird deaths. These were conducted in different locations across the country.

Despite the difficulty in getting the right number, efforts like this one are important, Wayne Thogmartin told Science News. An ecologist at the U.S. Geological Survey in LaCrosse, Wisc., he did not work on the study. He said “imperfect science” may inspire other researchers to get involved and help improve the data.

In the meantime, there are ways to save the birds. If a particular window seems to claim more bird lives than most, attach a piece of reflective tape. This signals the bird to steer clear.

Power Words

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ornithology  The scientific study of birds.

skyscraper  A very tall building.