St. Louis sits along the Mississippi Flyway — a corridor used by about 40% of the nation’s migratory waterfowl — which makes the city’s glow especially costly. Birds often mistake the glow of the arch for a celestial object, becoming confused and disoriented. That’s why the Gateway Arch is going dark for the month of September. The National Park Service has been doing this every spring and fall since 2002, and it has helped save millions of birds. About 80% of birds migrate overnight. They prefer migrating at night because they can use celestial objects like stars for guidance, and there are also fewer predators awake and active in the overnight hours. The 630-foot-tall Gateway Arch is typically lit by 76 exterior lighting units that illuminate the monument at night. It's also capable of displaying 44 different color variations for special events and awareness campaigns.
