Total birds

2900 Birds crossed Arapahoe County this night (est.)

Starting: Mon, Apr 19, 2021, 7:40 PM MDT

Ending: Tue, Apr 20, 2021, 6:10 AM MDT

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Peak migration traffic

12,600 Birds in flight (est.)
Direction: WSW
Speed: 1 mph
Altitude: 2,100 ft
Recorded: Tue, Apr 20, 2021, 12:00 AM MDT
Not all birds may fully cross a region in one night, resulting in a greater number of birds in flight than total birds crossed.

Here's what happened across this night...

Birds in flight

Birds usually begin to migrate 30 to 45 minutes after sunset, with the greatest number in flight two to three hours later.

Flight direction and speed

Birds tend to migrate northward in spring, but seasonal timing, weather, and geography alter their flight directions and speeds.


Migrating birds regularly fly up to 10,000 feet above ground, although seasonal timing and weather conditions dramatically impact their distributions.

Note: Direction, speed, and altitude may be unreliable when the number of birds in flight is low. In such cases, chart data may include gaps.

And across this season...

Birds in flight (nightly avg.)

During spring migration, most birds pass through the contiguous U.S. from mid-April to mid-May.

Total birds crossed

This is the estimated total number of birds that have fully crossed this region. The curve rises quickly during peak migration and levels off at the end of the season.

Expected nocturnal migrants

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Nocturnally migrating species most likely to arrive or depart this region at this time of year, based on frequency of observations


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Cornell Lab of Ornithology
Colorado State University
UMass Amherst