DENVER (KDVR) — Millions of birds are migrating over Colorado this weekend as they travel south for the winter.

On Friday night, nearly 1.2 million birds were estimated to have crossed over the state, with nearly 3.3 million in flight, according to BirdCast. An estimated 56 million birds have already crossed over Colorado this season.

“September is a big month for fall migration, with billions of birds (many of them young of the year) heading southward toward wintering grounds,” the Cornell Lab of Ornithology said.

Even Denverites may see the migration — more than 2,000 birds per kilometer are expected over the city each night this weekend.

Bird migration is usually at night

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

Bird migration usually begins 30-45 minutes after sunset and peaks around 2-3 hours later. It could make for great birdwatching, and it also poses an opportunity for the public to help keep the traveling flocks on their paths.

The migration is “a good time to turn off nonessential indoor and outdoor lights to avoid disorienting birds,” the Cornell lab posted on Friday. “Sadly, light pollution and unsafe windows contribute to the hundreds of millions of birds estimated to die in building collisions in the U.S. every year.”

More than 80% of birds migrate at night. Lights at night can throw birds into confusion and exhaust them, leaving them vulnerable and depleting their energy, according to the National Audubon Society.

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

Which birds are migrating over Colorado?

Bird watchers are encouraged to look at the skies both morning and night to observe the migrant birds in their areas. Contribute to the scientific understanding of birds by submitting findings to the eBird app.

BirdCast lists several nocturnal migrant bird species expected over Colorado:

  • Wilson’s Warbler
  • Eastern Kingbird
  • Lark Sparrow
  • Blue Grosbeak
  • MacGillivray’s Warbler
  • Western Kingbird
  • Solitary Sandpiper
  • Baird’s Sandpiper
  • Warbling Vireo
  • Olive-sided Flycatcher
  • Western Wood-Pewee
  • Black-headed Grosbeak
  • Sanderling
  • Clay-colored Sparrow

See how many birds are expected over your city here.