As snow melts from Colorado’s peaks and trees and flowers begin to blossom in the valleys, dozens of species of birds are migrating north, flying thousands of miles as part of an annual odyssey.

To aid these migratory birds in their great journey, residents throughout the Rocky Mountains are taking the simple step of turning off outdoor lights to aid the millions of feathered friends flying by night.

“Colorado is a major migration pathway, and we will have, in just the next few weeks, millions of birds flying over,” Roaring Fork Audubon vice chair Dalia Malone said. “So, turning out your lights at night is really critical. It is really interesting to understand that most of our songbirds, like small, little warblers, migrate at night.”

Malone described the springtime migration in Colorado as “one of our most spectacular events” because birds, many weighing only a few grams, are flying thousands of miles to return to their nesting grounds in the state. Even more are passing through the night sky on their way home to locales elsewhere in North America.

Already this spring, more than 7 million birds flew through Colorado’s skies in a single night, on May 1, according to BirdCast.info, a website that tracks bird migrations using radar. And the migratory season is only just getting underway.

Migratory birds, many of which are beloved for the sweet songs they sing in the background of spring and summer, “have made it through natural, dramatic events” like storms and highwinds, Malone said. But as researchers have developed a better understanding of bird migration in recent decades, it has become increasingly clear that migratory species also face “human barriers,” like light pollution, which can disrupt their intrepid journeys, she said.

Luckily, birders in Colorado and beyond are working together to raise awareness for the millions of birds that migrate by night, calling on their neighbors and governments to turn off lights or at least be thoughtful about outdoor lighting.

In Frisco late last month, about 30 local residents gathered at Slopeside Hall to listen to a talk by Richard O’Brien, a volunteer with the nonprofit DarkSky Colorado, and the founder of Lights Out Colorado, which is helping to spread the word about nocturnal bird migration.

Like many Coloradans, O’Brien is an outdoor enthusiast, who said he is “happier and healthier” when he spends time outside. Describing himself as “really just a guy who likes to look at birds,” he said that when he learned that bird populations in North America, and throughout much of the world, have seen steep declines in recent decades, he wanted to help do something about it.

One study, published by the Cornell Lab of Ornithology, found that North America has lost nearly 3 billion birds in the last 50 years, a nearly 30% decline in the overall population of birds across the continent.

About 80% of birds that migrate do so at night, often using the stars to navigate, O’Brien said. But human lights can “distract and disorient” these migratory species, with research showing that birds turn toward sky glow, and circle bright, unshielded light, leading them off course and wasting precious energy during their long journey, he said.

“So what can we do? What practical changes in our lighting can we make to help birds?” O’Brien asked. “First of all, the thing we can all do even tonight is instant. Turn off unnecessary outdoor lights. It’s really easy to turn off unnecessary lights. Just flip the switch and leave it off during bird migration season.”

That’s what Lights Out Colorado is helping state residents understand. By signing up for the group’s email list at Lights-Out-Colorado.DarkSkyColorado.org, residents can get email notifications when birds are forecast to migrate, reminding them when to turn off outdoor lights.

But the work doesn’t stop there, O’Brien said. In the longer term, residents can be more thoughtful about their outdoor lighting, hooking it up to timers or motion detectors, so it can remain helpful but isn’t on all the time. Choosing yellowish-white lighting or amber-colored lighting, rather than bluish-white lighting, can also make outdoor lighting less disturbing to wildlife, he said.

Coloradans can also “downcast” their lighting, applying shields so that it doesn’t shine up into the night sky unnecessarily, and they can advocate for local governments and businesses to also take action, O’Brien said. 

“In my experience, people often don’t realize how bad (outdoor lighting) is. It is often a matter of patience to get through to them. If you go to them angry and say you’ve got to change things right now, they’re going to dig their heels in,” O’Brien said. “But if you get them on your side then good things happen. That is why we need everybody. This is a grassroots thing. Everybody needs to talk to their neighbors about this.”

In Steamboat Springs, David and Tresa Moulton, who lead outings with the Yampa Valley Birding Club, say they leave their outdoor lighting off throughout the year, only turning it on to get safely to and from their home. 

Those who love birds can also help by keeping domesticated cats indoors, since outdoor cats kill approximately 2.4 billion birds every year, according to the American Bird Conservancy, and leaving dead trees on their property, when possible, because birds enjoy snacking on the bugs in snags, the Moultons said.

“Spring is an exciting time for birders,” Moulton said. “Birds just keep showing up and you just keep seeing more and more. The plants are coming back and the birds and everything are coming back. It’s a great rejuvenation. It gets all the blood stirring again to see everything coming back alive and emerging from the winter blanket of snow. Every day can be a little different as hundreds of thousands of birds migrate, and some stop.”