Windows may kill up to 988 million birds a year in the United States

Between 365 million and 988 million birds die from crashing into windows in the United States each year, according to the latest estimate.

That might equal 2 to 10 percent of the (admittedly uncertain) total bird population of the country.

The biggest share of the deaths comes not from glass massacres at skyscrapers but from occasional collisions with the nation’s many small buildings, says Scott Loss of Oklahoma State University in Stillwater. “It’s death by a million nicks.”

Low-rise buildings four to 11 stories tall account for about 56 percent of deaths in the new estimate, Loss and his colleagues report in the February Condor: Ornithological Applications. Residences that are one to three stories tall make up around 44 percent, with skyscrapers representing less than 1 percent.

Any given small building kills only a few birds each year versus the 24 expected to die annually at a single skyscraper. But the United States has about 15.1 million low-rises and 122.9 million small residences, and only about 21,000 skyscrapers. Loss applauds efforts to make skyscrapers bird-friendly, but cautions that protecting birds takes a broader effort.

Some species — many of them Neotropical migrants — appear especially vulnerable to the deceptions of windows, Loss and his colleagues find. Among the possible reasons are the risks of disorientation from artificial lights for birds on long-haul migrations at night. Compiling data from all kinds of buildings, the team found that Anna’s hummingbirds, black-throated blue warblers, ruby-throated hummingbirds, Townsend’s solitaires and golden-winged warblers topped the risk list.

Among those birds, conservationists have already flagged the golden-winged warbler because of its steep population decline in recent decades. Other troubled species nationally that rank high in vulnerability to window crashes include painted buntings, wood thrushes and Kentucky warblers.

It’s these already distressed species that worry Loss the most. For individual species with dwindling numbers, he imagines window kills might affect population trends.

The estimate puts windows, just behind cats, as the second-largest source of human-related menaces that kill birds directly (SN: 2/23/13, p. 14). From what Loss knows of estimates of other perils to birds such as wind turbines and vehicle kills, he says, “nothing else comes close.”

There’s no nationwide reporting of birds thumping into glass or succumbing to a paw, so estimating death tolls has long been difficult and controversial (SN: 9/21/13, p. 20). The new estimate of mortality from windows, based on statistical analysis of 23 local studies, comes close to an old estimate (100 million to 1 billion) that had been derided for its simple, back-of-the-envelope approach. “We were a little surprised,” Loss says.

There are plenty of uncertainties in extrapolating from small, diverse, local studies, particularly in trying to estimate overall species vulnerabilities, says Wayne Thogmartin  of the U.S. Geological Survey in LaCrosse, Wis. But even such “imperfect science” has value, he says. For one thing, it may inspire people to start filling in gaps in data.

The total for window kills isn’t the whole story, though, says ornithologist Daniel Klem Jr. of Muhlenberg College in Allentown, Pa., who did the earlier calculation: “The moral imperative of preventing even one unwanted and unintended death of these utilitarian and aesthetically pleasing creatures is, or should be, compelling enough.”