More than a billion birds are killed in window collisions each year

A new study has provided clear evidence that significantly more birds are killed by window collisions than previously thought.

The researchers noted that building collisions impact hundreds of bird species, killing between 365 million and one billion birds a year in the United States alone. 

The study was led by experts at the Fordham University, the NYC Bird Alliance, Inc, and the Max Planck Institute for Geoanthropology. 

Collisions are a leading source of bird mortality 

“Even common avian species in North America are declining, so identifying and addressing threats to bird populations is critical for the conservation of not only individual species, but the ecosystems they belong to,” wrote the study authors. 

“Collisions with buildings are a leading anthropogenic cause of death for birds in the United States, with major collision events garnering national attention and, as of 2024, 25 U.S. cities or states have passed legislation that requires new buildings to be bird-friendly.”

Severe injuries and near-instant deaths

Previously, scientists have shown that windows are a major hazard for a great variety of birds, which often don’t notice them and fly directly into them. 

In some cases, these collisions lead to a “stunning effect,” with birds crashing to the ground, only to recover moments later and fly away. In other cases, the impacts lead to severe injuries or near-instant-deaths.

“Building collisions are a leading threat to wild birds; however, only those that are found dead or fatally wounded are included in current mortality estimates, with injured or stunned birds largely assumed to survive long-term. Avian building collision victims are often brought to wildlife rehabilitators for care, with the hopes they can be released and resume their natural lives,” the authors explained.

Bird deaths are underestimated 

According to the experts, although scientists have previously estimated that hundreds of millions of birds die annually in the United States due to such collisions, the number may in fact be much larger. 

While previous window collisions calculations are derived by adding up counts of dead birds found on the ground, they fail to take into account the cases of birds surviving the initial collisions but dying later due to injuries.

In order to estimate the number of birds dying after the initial impacts, the scientists examined data from eight states on 3,100 avian-window collisions between 2016 and 2021. 

Brain injuries are common

The analysis revealed that most birds that were discovered and treated for their injuries – about 60 percent –  still died later on, mainly due to brain injuries.

“The number of admissions varied by season; fall migration had the highest number of cases and winter had the least number of cases, and summer having the lowest release proportion and winter having the highest. The most common reported injury was head trauma and concussion,” the authors explained.

Thus, while it is not uncommon for a bird to wake up after a collision and fly away apparently unharmed, they nevertheless seem to experience the same type of symptoms happening to people after car accidents, such as brain swelling, which nearly always leads to death in birds. 

A new perspective on bird-building collisions 

Taking this data into account, the experts estimate that the total number of bird deaths after window collisions may in fact significantly exceed one billion a year in the United States alone.

“This study reports different data than carcass studies and views bird-building collisions from the perspective of surviving victims to explore longer-term effects of these collisions on mortality,” noted the researchers.

The experts recommend increased communication and collaboration between wildlife rehabilitators and conservation researchers to better understand building collisions and how to respond to this leading threat to wild birds. 

“These findings, along with our estimate of delayed mortality, suggest that overall collision mortality estimates based on carcass collection far exceed one billion birds in the U.S. each year,” the authors concluded.

The study is published in the journal PLOS ONE.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–