Colorado residents are being called on to dim their lights to aid the millions of birds flying over the state each night as a massive annual migration takes off.

Every year, billions of birds migrate north in the spring and south in the fall, most of them traveling by night, often navigating by starlight, according to the National Audubon Society. However, artificial lights from humans can disorient the birds, throwing them off their migration path and sometimes causing them to collide with buildings or windows.

“Migrating at night allows birds to avoid certain predators,” Audubon Society bird-friendly building program manager Connie Sanchez said in a news release. “It’s also more energy efficient. Birds can conserve more energy on their migratory route by flying in cooler temperatures in the dark.”

During the migration, the Audubon Society and other conservation groups are asking residents to take simple actions to help protect the birds traveling at night: Shield outdoor lights to prevent light from being emitted upwards and turn off lights as early as possible during the bird migration season, which lasts from late August through September.

In Colorado, almost 6 million birds flew over the state Wednesday night, Sept. 4, according to BirdCast, a real-time predictor of bird migrations. An estimated 193,000 birds flew over Summit County on Wednesday night. 

Many birds are drawn to the artificial lights of buildings found along migratory pathways, according to the Audubon Society. Research estimates that building collisions kill as many as 1 billion birds in the U.S. each year.

It’s not just skyscrapers that birds are crashing into. Nearly half of birds that die from flying into buildings in the U.S. collide with homes and smaller buildings, according to the Audubon Society.

Residents can help prevent birds from colliding with structures by turning out their lights while the annual migration is taking place. For more information visit Audubon.org/our-work/cities-and-towns/lights-out.

“Many people don’t realize the threat of residential buildings to birds,” Sanchez said. “Instead, they’ll often think of big cities and urban areas; however, it’s a widespread issue that’s happening everywhere, at all times of the day, at all times of the year.”