Over a billion birds die after striking U.S. buildings each year — maybe many more

The majority of birds that are injured or stunned hitting buildings don’t recover, a new U.S. study has found, which could push estimates for the number of birds killed flying into buildings far above 1 billion a year.

Hitting buildings is a significant threat to wild birds. Estimates of U.S. mortality rates from building crashes are based only on the number of birds found dead or mortally injured, not those that die later of their injuries.

In the latest study, published in the journal PLOS One on Wednesday, the researchers looked at birds that were taken to wildlife rehabilitation centers to assess their long-term survival rates.

The researchers studied cases from the northeastern United States between 2016 and 2021. They examined the records of over 3,100 building crashes with 152 bird species and found that about 60% of the birds taken to rehabilitation centers died, either by succumbing to their injuries or euthanasia.

“Our findings are especially concerning when considering that death rates in the wild may be even higher than 60% if collision victims are not found and treated by rehabilitators,” the study’s authors wrote.

Many of those that survived required multiple days to recover, “a luxury that is not offered to most birds outside of rehabilitation,” they noted.

Mourning doves, American robins, northern cardinals, cooper’s hawks and gray catbirds were the most commonly found species in the study. The most common form of injury was head trauma and concussion. The vast majority appeared otherwise healthy before the injuries suffered in the building crash, the researchers said.

The number of birds taken to rescue centers varied by the season, with the highest number being recorded in the fall – a time when birds are migrating. Light pollution is a particular problem for migratory species, luring them to big cities and urban areas where they’re at a higher risk of hitting buildings, the researchers said. In an incident last year, nearly 1,000 birds died in one day after striking a Chicago building during their migration. The dead birds included warblers, woodcocks and sapsuckers.

The study’s authors noted that a number of U.S. cities and states have passed regulations requiring new buildings to be more “bird-friendly” amid a growing recognition that birds are an important part of the ecosystem, from controlling insects to helping disperse seeds.

Windows and glass are one of the biggest problems because birds have poor depth perception and aren’t able to detect architectural features like window frames.

It isn’t just skyscrapers: Crashes also occur frequently around individual homes, especially those with bird feeders, the researchers said.

Wildlife biologists are testing ways of making windows more visible to birds, or making them appear as if they are too small to fly through, according to the Pennsylvania-based Green Building Alliance.

Building experts say the easiest way to make windows bird-friendly is to add designs or decals to window pane surfaces. Colored, frosted or stained-glass windows are also more noticeable to birds. Some buildings use balconies, grilles or shutters to reduce the amount of reflective glass.