Coloradans are being asked to dim their lights to aid millions of birds flying over the state each night amid the annual fall migration.

Every year, billions of birds migrate north in the spring and south in the fall. Nearly 80% of birds that migrate do so at night, often navigating by starlight, according to the National Audubon Society. However, artificial light from humans can disorient the birds, throwing them off their migration path and sometimes causing them to collide with buildings or windows.

“Our artificial lights are a big problem for them,” Richard O’Brien, the chair of Lights Out Colorado, said in an email. “They have amazing journeys, sometimes thousands of miles to South America. It’s time to remind our neighbors: Turn out unneeded outdoor lights, each night for at least the next month.”

During the migration, Lights Out Colorado — a program supported by Dark Sky Colorado and the Denver Audubon — asks 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.

In Colorado, the peak bird migration starts on Sept. 1 and continues through Sept. 24, according to BirdCast, a migration forecast tool created by The Cornell Lab of Ornithology, Colorado State University and University of Massachusetts Amherst.

Already, on the night of Aug. 28, an estimated 3.5 million birds were in flight over Colorado — and the migration is only expected to increase in the coming days and weeks, according to BirdCast. 

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

And it’s not just skyscrapers 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.