More than 31 Million Birds Have Crossed El Paso County This Fall

Radar estimates from the Cornell Lab of Ornithology’s BirdCast project put the number of birds that have flown over El Paso County since Aug. 1 at about 31.2 million, a fifth of them on one night in September. The usual peak has passed. About a fifth of the fall flight is still ahead, and most of what helps the birds get through costs a household nothing.

The fall bird migration over the Tri-Lakes happens after dark and overhead, which is why a movement of this size goes largely unremarked. BirdCast, a project of the Cornell Lab of Ornithology and three partner universities, turns the returns from the country’s weather radars into estimates of how many birds are aloft, how fast they are moving and in which direction. Its dashboard for El Paso County estimated that 2.26 million birds crossed the county on the night of Tuesday, Sept. 29. At 12:50 a.m. about 1.1 million were in the air at once, heading south-southwest at 24 mph, roughly 2,600 feet above the ground.

That night brought the season’s total to an estimated 31.2 million birds, 1.7% above BirdCast’s historical average for the date. The figures cover the county as a whole; BirdCast publishes nothing finer, and the Tri-Lakes sits at the county’s northern end. By that measure 2026 is an ordinary year, delivered irregularly.

Six nights, half the birds

On the night of Sept. 14 an estimated 6.13 million birds crossed the county, a fifth of everything recorded so far this season. At the night’s busiest moment they were moving south at 43 mph over the ground, a speed that, as BirdCast notes, includes whatever wind was behind them. Six nights out of 60 account for 52.8% of the season’s total. On 24 nights fewer than 100,000 birds crossed.

The pattern is the rule rather than a quirk of this year. BirdCast says about 10% of a season’s nights carry about half of its traffic. Its explanation is weather: birds decide night by night whether to fly, and a tailwind lets them cover more distance for less energy.

What is left, and when

BirdCast defines a county’s peak period as the shortest stretch in which half of a season’s traffic has historically passed. For El Paso County that window, calculated from radar records for 1995 to 2018, runs from Aug. 31 to Sept. 27. Purdue University’s AeroEco Lab, a BirdCast partner, lists Sept. 6 to Sept. 30 for Colorado Springs. By either reckoning the peak is over.

What remains is smaller and still substantial. In BirdCast’s historical average, 81% of the fall’s birds have crossed the county by Sept. 29 and 19% have not, which in an average year is about 7.2 million birds. Half of that remainder has passed by Oct. 8 and nine-tenths by Oct. 21. The average night sends about 500,000 birds over the county on Oct. 1, about 190,000 on Oct. 15 and about 25,000 on Halloween.

Averages hide the nights that matter. The Sept. 29 flight, the second-largest of the season, came two days after the historical peak window closed. BirdCast’s forecast for Monument, checked on Wednesday evening, called for medium traffic that night, low on Thursday and medium on Friday, with no high-migration alert. The project sends email alerts ahead of the heaviest nights, and its dashboard runs live from sunset to sunrise through Nov. 15.

Radar cannot tell one species from another. For that BirdCast turns to the checklists birders file on eBird, whose page for the county lists 428 species. On BirdCast’s list of likely night migrants for the county, chipping sparrows appear on 24% of checklists in the first week of October and orange-crowned warblers on 12%. Wilson’s warblers, on 36% of checklists in the second week of September, drop to 9%. Sandhill cranes, nearly absent from September’s lists, turn up on 4% to 5% in the first half of October. BirdCast describes the list as experimental. White-crowned sparrows, which are not on it, are on 33% of the county’s eBird checklists in the first week of October, more than any species that is.

What residents can do

The hazards are mostly at house height. A 2014 review in the journal The Condor estimated that collisions with buildings kill between 365 million and 988 million birds a year in the United States, with 44% of the deaths at residences, 56% at low-rise buildings and less than 1% at high-rises. The risk sits with houses and low buildings, which is to say with places like the Tri-Lakes.

Lights. Artificial light draws night migrants toward buildings and glass. BirdCast asks for nonessential lights off from dusk to dawn through Nov. 30. Purdue’s lab and the Aiken Audubon Society, the Audubon chapter for the Pikes Peak region, ask for 11 p.m. to 6 a.m. Lights Out Colorado, a statewide campaign, says to switch off as early in the evening as possible, and BirdCast’s numbers for the county favor the earlier hour: on Sept. 29 about 412,000 birds were overhead at 8:20 p.m. and more than 600,000 by 8:50. Lights required for safety stay on; the target is decorative and unnecessary lighting.

Fixtures. For lights that stay lit, BirdCast, Lights Out Colorado and Aiken Audubon give the same advice: aim them down, shield them, add motion sensors and choose warm bulbs. BirdCast and Lights Out Colorado put the ceiling at 3,000 kelvin. The draft rewrite of the El Paso County Land Development Code, as the Independent reported in August, would require covered outdoor fixtures to fall generally below 3,001 kelvin, a rule the current code lacks.

Windows. Birds do not see glass as solid, and by day they fly at what it reflects. BirdCast’s guidance is to put a pattern on the outside surface, with markers no more than 2 inches apart, starting with any window that has already been hit and those near feeders, birdbaths, gardens and trees. Tempera paint, paint markers and cord hung in front of the glass all work and come off again. Turning lights out, BirdCast says, does not by itself prevent collisions.

Cats. Cats kill more birds than glass does. The Cornell Lab puts the toll at more than 2.4 billion birds a year in the United States and Canada, the largest human-caused source of bird deaths apart from habitat loss. Its remedy is to keep cats indoors or in an enclosed outdoor run.

Feeders. The instinct to put out seed for the travelers is the one to resist. Colorado Parks and Wildlife says bird feeders “are only appropriate in the winter months, when bears are hibernating,” and suggests nest boxes, flower pots and a birdbath in the meantime. The point is a live one in the Tri-Lakes this fall. Bears and trash have been before Palmer Lake’s trustees since August; the town’s police chief told them on Sept. 24 that enforcement covers bird feeders along with other attractants, and a workshop on the subject was tentatively set for Oct. 12.

Yards. For the birds that come down to rest and feed, the yard matters more than the feeder. Audubon Rockies runs a program, Habitat Hero, for bird-friendly gardens of native plants in Colorado, and the Cornell Lab’s list of household actions adds cutting back on pesticides.

A stunned bird. A bird that has hit a window and is still alive may need only time. Aiken Audubon’s advice is to put it in a covered cardboard box in a cool, safe place, give it neither food nor water, and take the box outside every 15 minutes to see whether it will fly. If it has not after a few hours, Colorado Parks and Wildlife keeps a list of licensed wildlife rehabilitators.

Those who would rather see the migration than read the radar have a chance on Saturday, Oct. 3, when Aiken Audubon leads a bird walk at Kettle Lakes, near the Air Force Academy’s south gate, from 8 to 11 a.m.; the chapter asks for an RSVP. Sightings filed on eBird feed the species lists BirdCast publishes.

BirdCast’s live feed for the season runs through Nov. 15. By its historical average, 99% of the fall’s birds will have crossed El Paso County by Oct. 27.

Sources & further reading

The Independent’s own coverage: Bears, Trash and a Town Hall, on why bird feeders are part of Palmer Lake’s bear debate · Palmer Lake Sets Bear Workshop, for the police chief’s remarks and the workshop date · El Paso County Is Rewriting the Rules the Buc-ee’s Fight Turned On, for the lighting rules in the county’s draft code

Migration data: BirdCast Migration Dashboard, El Paso County, Colorado · BirdCast, how the dashboard works · BirdCast, migration periods and the county peak-period table · BirdCast FAQ · BirdCast migration alert for Monument · Purdue AeroEco Lab, Colorado forecast and seasonal timing

Lights and glass: BirdCast, Turn Lights Out · BirdCast, Treat Glass · Lights Out Colorado · Aiken Audubon Society, Lights Out for Birds · Loss and others, “Bird–building collisions in the United States,” The Condor, 2014

Cats, feeders and yards: Cornell Lab of Ornithology, Seven Simple Actions to Help Birds · Colorado Parks and Wildlife, Feeding Wildlife · Audubon Rockies, Habitat Hero

Local help: Aiken Audubon Society, What to Do with an Injured Bird · Colorado Parks and Wildlife, licensed wildlife rehabilitators · Aiken Audubon Society, Kettle Lakes bird walk, Oct. 3

Photographs: Chipping sparrow, Paul Danese (CC BY-SA 4.0) · Orange-crowned warbler, Fyn Kynd (CC BY 2.0) · Wilson’s warbler, Dario Taraborelli (CC0) · White-crowned sparrow, Wolfgang Wander (CC BY-SA 3.0), all via Wikimedia Commons

Methodology

Every bird figure in this article is a BirdCast estimate drawn from weather radar. None is a count. BirdCast’s algorithms remove weather from the radar signal, convert what remains into a density of birds and integrate it over area and altitude. BirdCast says accuracy falls in rain, that radar in mountainous regions can miss birds flying under the beam, and that radar cannot distinguish species.

The Independent read the El Paso County dashboard on the evening of Wednesday, Sept. 30, 2026, and saved the data the page was built from. The season total is BirdCast’s cumulative “total birds crossed” through the night of Sept. 29, the last complete night; Sept. 30 was in progress and is excluded. Nightly figures are the night-to-night change in that cumulative series, and the Sept. 14 and Sept. 29 figures match the nightly totals on the dashboard’s own pages for those dates. Shares of the season, the count of nights and the dates by which portions of the historical remainder have passed are the Independent’s calculations from BirdCast’s series. A night runs from local sunset to the following sunrise, and BirdCast’s fall season from Aug. 1 to Nov. 15.

“Historical average” is BirdCast’s own comparison series; its documentation describes the dashboard’s historical data as covering 2013 to 2021. The county peak period comes from a separate BirdCast table built on radar records for 1995 to 2018. Species percentages are the share of eBird checklists reporting a species in a given week of the year, as shown on the dashboard, which divides the year into 48 periods, four to a month. The white-crowned sparrow figure comes from eBird’s own bar chart for the county, read the same evening. The three-night forecast for Monument was read on the evening of Sept. 30 and will have changed. The collision figures come from the abstract of the 2014 paper. The four bird photographs come from Wikimedia Commons under the licenses named in their captions; none was taken in El Paso County, and they appear resized and otherwise unaltered. No one was interviewed for this article.