DENVER (KDVR) — Every night from late summer into fall, millions of birds are flying over the Rocky Mountains in search of a warmer climate, and the lights in Colorado can disorient their path.

More than 3 million birds flew across the Centennial State Sunday night, according to estimates from BirdCast. Bird migration overnight in the millions is not uncommon during peak season, which a spokesman for Audubon Rockies said we are currently in.

“Each fall migration, approximately 250 million birds migrate through or over Colorado,” said Evan Barrientos with Audubon Rockies. The nonprofit stationed in Fort Collins focuses on protecting birds around the world and in Colorado.

More than 4 billion birds in North America fly south in the fall. An estimated 316 to 400 million birds were expected to migrate south over the U.S. each night over last weekend.

Bird migration usually begins 30 to 45 minutes after sunset and peaks around 2 to 3 hours later. But the light that communities keep on at night can confuse birds on their long journey.

“Cities along the Front Range like Denver and Colorado Springs are particularly problematic for migratory birds,” Barrientos said. “It’s not just a Front Range thing. Birds migrate across the entire state.”

Lights at night can throw birds into confusion and exhaust them leaving them vulnerable and depleting their energy during a journey of thousands of miles, according to Barrientos.

“When they’re migrating at night, they use the stars and moon to navigate,” Barrientos said. “That’s why artificial light from our buildings disorients them.”

The Colorado State University Aeroeco Lab estimates 6.1 million birds will fly over Colorado on the night of Sept. 12, with roughly 11,000 flying over the Denver metro.

How to help reduce light pollution

The National Audubon Society has tips for helping keep migrating birds on their path:

  • Turn off exterior decorative lighting
  • Extinguish pot and flood-lights
  • Reduce atrium lighting wherever possible
  • Turn off interior lighting, especially on higher stories
  • Substitute task and area lighting for workers staying late or pull window coverings
  • Use shielding and aiming so that no light is emitted upwards
  • Install automatic motion sensors and controls wherever possible
  • When converting to new lighting assess the quality and quantity of light needed, avoiding over-lighting with newer, brighter technology