The lights illuminating the Gateway Arch will go dark for two weeks beginning Monday.

The decision is part of a biannual effort to avoid disrupting bird migrations along the Mississippi Flyway — a critical route used by more than 300 North American bird species. Light pollution from upward-facing spotlights can disorient birds that migrate at night and cause them to collide with buildings.

“We’ve often been asked, ‘Why do you bother when you’re in a big city that’s throwing off all this light?’” said Gateway Arch National Park Deputy Superintendent Frank Mares. “It’s because the Arch is possibly the tallest thing any bird will come upon, right on the river.”

Thousands of birds travel along the Mississippi Flyway each year from breeding grounds in Canada to overwintering areas in the southern U.S.

As its name suggests, the flyway hugs the banks of the Mississippi River, stretching from Canada to the Gulf of Mexico.

Many songbirds and waterfowl species migrate at night, potentially to avoid predators. But technological advances have brought stronger, brighter lights that can disorient these nighttime migrants.

In recent years, cities and residents have also installed upward-facing spotlights to illuminate buildings and other structures.

Mitch Leachman, program director for the St. Louis Audubon Society, said these spotlights can be particularly confusing to nighttime-migrating birds. In some cases, birds appear to be attracted to and “trapped” by artificial light sources.

“You can see birds flying in the light beam,” said Leachman. “They’re disoriented, and they can’t figure out how to get out.”

Although disorientation from artificial light may not directly cause bird deaths, Leachman said it can stress migrants during a taxing period of their lives.

“These birds are traveling hundreds — and in some cases thousands — of miles. They may be losing 20 to 30 percent of their body weight in doing so,” Leachman said. “[It’s] an entirely vulnerable part of their life.”

At the request of the St. Louis Audubon Society, park officials decided to turn off the Arch lights temporarily in 2001, the same year the lights were installed.

They now shut off the lights for two weeks during peak migration periods in May and September.

In August 2018, the Gateway Arch exterior-lighting system underwent a $1.2-million renovation, funded by the Gateway Foundation of St. Louis. As part of the project, the 44 original light fixtures were replaced with 26 xenon skylights outfitted with custom lenses.

Mares said the new lights provide a “laser-like focus” on the Arch, which he hopes will benefit migrating birds.

“The lights are brighter, but there’s much less light overspray than there used to be,” Mares said. “There’s less light pollution above and around the Arch that could disorient a night-migrating bird.”

But to be safe, officials will continue turning off the lights for brief periods to protect birds in the spring and fall.

The Arch lights will be turned back on beginning Oct. 1.

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