The saddening statistic comes from a 2014 study led by Scott Loss, an assistant professor of global change ecology and management at Oklahoma State University. More recently, a new study from the Cornell Lab of Ornithology found that Chicago, Houston, and Dallas are the top three U.S. cities that are especially deadly for birds, and artificial light at night is a contributing factor.

“Chicago, Houston, and Dallas are uniquely positioned in the heart of North America’s most trafficked aerial corridors. This, in combination with being some of the largest cities in the US, make them a serious threat to the passage of migrants, regardless of season,” said Kyle Horton, the lead author of the study, in a statement published by the Cornell Lab of Ornithology.

“Now that we know where and when the largest numbers of migratory birds pass heavily lit areas we can use this to help spur extra conservation efforts in these cities,” added study co-author Cecilia Nilsson.

Making changes like incorporating bird-friendly building design or simply turning off unnecessary lights at night when birds are likely migrating can play a role in mitigating the widespread issue. The Cornell Lab also used the data they gathered to create the website Birdcast, which provides migration forecasts.