The Chicago Megaflight of 5 October 2023

by Nathan Goldberg, Marky Mutchler, and Jacob Drucker

October 16, 2023

5 October 2023 will be known in the Chicagoland area, and perhaps all of North America, as the date of one of the most impressive morning flights in recent memory. Weather conditions on the night of 45 Oct indicated the potential for a huge flight. Migration through northern Illinois and the surrounding area had been stalled by 15 days of southerly winds and 80+°F (27+°C) temperatures. A cold front finally arrived from the north in the predawn hours of 5 Oct and gave millions of birds the boost they needed to finally head south.

When the north winds reached Chicago, they were accompanied by rain and low clouds, as is typical of a strong cold front with adequate moisture. Birds don’t want to fly through rain and into unfavorable winds, so they started to “pile up” along and behind the front, which arrived just before dawn in Chicago. That’s when hundreds of thousands of birds, balancing survival and the need to move south, started to become detectable by observers on the ground.

As dawn approached, birders realized that the air was filled with the flight calls of migratory birds. Marky Mutchler, one of the authors of this post, was roused from her sleep by the nocturnal flight calls right outside their open window. Early risers began sharing their first impressions in the Cook County channel of the “Illinois Birding” Discord server at dawn:

“It’s insane the numbers of songbirds around. Incessant chip notes everywhere”
–Vinod Babu

“There are hundreds of small passerines moving south from my window near Lake Shore Drive.”
–Howard Blum

“Counting at least 100 [warblers] per minute.”
–Audrey Carl

“Insane migration happening right now, look up!”
–Tamima Itani

“I’m in Ravenswood and i can see hundreds of passerines streaming over. No binoculars or anything.”
–Dustin Weidner

As the morning went on, more observers ventured out to bear witness to the spectacle. They quickly stationed themselves at various local migrant traps and did their best to make sense of the unfolding chaos. It was clear this would be a monumental day.

With a seemingly never-ending cascade of birds over the Chicago region, birders diligently recorded the numbers as best they could by conducting rate counts throughout the morning.

Rate counts help birders estimate huge numbers of birds as accurately as possible. To conduct a rate count, you watch a fixed patch of sky for one minute and meticulously tally all the birds flying through the field of view. If multiple observers are at the same location, each observer counts birds in a separate section of sky simultaneously for one minute. Multiplying the number of birds counted per minute by ten allows for an estimation of bird numbers across ten-minute intervals. Adding together the ten-minute totals derived from one-minute rate counts yields an estimated total number of birds across the entire observation period while taking into account variation in the ebb and flow of the flight. Rate counts are invaluable for quantifying high-magnitude migration events. Similar methods are used by the BirdCast project to estimate the number of birds crossing a region on a given night using weather surveillance radar data.

By day’s end, there were a few standout tallies (all of which were estimated using rate counts) submitted to eBird:

Burnham Park–Promontory Point, Chicago, Cook Co, IL
Observers: Marky Mutchler, Jacob Drucker, Lila Fried, and Woody Goss
~187,000 birds, including over 176,000 warblers.

Wrigleyville neighborhood, Chicago, Cook Co, IL
Observer: Nathan Goldberg
~172,000 birds, including over 120,000 warblers.

Fort Sheridan Forest Preserve, Lake Co, IL
Observer: Adam Sell
~34,000 birds, including over 23,000 warblers.

Marquette Park, Gary, Lake Co, IN
Observer: Will Keller
~13,000 birds, including over 12,500 warblers

Over 140 species were reported in Cook County, Illinois via these and other eBird checklists on 5 Oct. The diversity and volume of birds can be attributed partly to the presence of both long- and short-distance migrants, as the tail end of Neotropical–Nearctic migration overlapped with the arrival of the first short-distance (intra-Nearctic) migrants. Most of the hundreds of thousands of warblers overhead were Yellow-rumps and Palms. Strong movements of Blue-winged Teal and Wilson’s Snipe along the Chicago lakefront were surprising, and an impressive raptor migration began in the afternoon after the rain passed. Every bit of undergrowth in the city seemed to be swarming with Ovenbirds, Northern Waterthrushes, Swainson’s and Hermit thrushes, and White-throated Sparrows.

One of many migrating Palm Warblers seen in Chicago on the morning of 5 Oct 2023. Photo © Nathan Goldberg.

A large flock of migrating Wilson’s Snipe in Chicago on the morning of 5 Oct 2023. Photo © Nathan Goldberg.

Where did all the birds tallied around southwest Lake Michigan come from? Birdcast estimated that 25.7 million birds passed over Dane County, Wisconsin the night of 45 Oct 2023. The center of Dane Co is only about 120 mi. (193 km) northwest of downtown Chicago. Northwest winds surely delivered many of these birds to Chicago the following morning, and thus the staggering totals reported by all observers in Chicago were likely undercounted.

For instance, Marky Mutchler, Lila Fried, and Jacob Drucker drove a 1.8 mi. (3 km) transect in Chicago, from Promontory Point to Washington Park, and conducted additional rate counts along the way. They estimated a rate of approximately 300 birds/minute across 328 yd. (300 m) intervals.

Applying this rate across the four hours of intense flight observed that morning yields an estimate of 720,000 birds flying across a single Chicago neighborhood. While not all these birds were directly observed, this is a similar number of birds estimated by observers in Tadoussac, Québec in May 2018.

This video captured by Woody Goss at Promontory Point in Chicago helps contextualize what birders experienced when they looked to the sky on the morning of 5 Oct 2023.

Woody Goss also recorded nocturnal flight calls in Chicago’s Lakeview neighborhood from 1:45-2:00 AM on 5 October using a 21c bucket mic from Bill Evans, which he had placed on his porch. His recordings capture the incredible rate at which birds flew over Chicago that morning. The first recording contains roughly 136 calls in 28 seconds (about 5 calls per second)!

Flight Calls recorded in the early morning of 5 Oct 2023 by Woody Goss.

Flight Calls recorded in the early morning of 5 Oct 2023 by Woody Goss.

Sadly, not all migrant birds made it safely through Chicago airspace that day. Annette Prince, director of Chicago Bird Collision Monitors, shared in a WGN radio interview with Lisa Dent that about 300 injured birds and 700800 dead birds were found in one square mile in downtown Chicago. Daryl Coldren of the Field Museum of Natural History shared the following images taken on 5 Oct of the almost 1,000 birds found dead after striking just one building on the Chicago lakefront—McCormick Place’s Lakeside Center. Lakeside Center has been monitored for bird collisions daily during migration for over 40 years, and the night of 45 Oct was the building’s worst ever for window strikes. Although Lakeside Center and McCormick Place in general have been the site of window strike deaths for decades, the City of Chicago and McCormick Place management have done very little to stop it.

Birds recovered outside McCormick Place in Chicago, Illinois on the morning of 5 Oct 2023. Photo © Daryl Coldren/Field Museum of Natural History.

Birds recovered outside McCormick Place on the morning of 5 October 2023. Photo © Daryl Coldren/Field Museum of Natural History.

Closeup of birds recovered outside McCormick Place on the morning of 5 October 2023.Photo © Daryl Coldren/Field Museum of Natural History.

The huge glass windows of McCormick Place’s Lakeside Center kill scores of migrant birds each year on the Chicago lakefront. 12 Oct 2023. Photo © Carl Giometti.

McCormick Place’s Lakeside Center sits along the lakefront and is seen here with the Chicago skyline in the background and Northerly Island Park in the foreground. 13 Apr 2022. Photo © Jacob Drucker.

While these images are distressing, they highlight both the importance of current “Lights Out” campaigns (including the voluntary Lights Out Chicago initiative) and the need for a full-fledged city ordinance requiring bird-friendly glass in new buildings. Chicago conservation organizations and the National Audubon Society are circulating a petition asking McCormick Place to turn the lights out at night. Turning out the lights at night during migration could save untold numbers of migrant birds at McCormick Place. It is our fervent hope that the City of Chicago will recognize the danger its buildings pose to migratory birds and implement some of the many proposed solutions. Please follow this link to add your signature to the petition.

Few if any morning flights of this magnitude have been documented in Chicago. Indeed, if the front had passed earlier (with rains reaching Chicago early in the evening of 4 Oct and clearing before dawn), this flight would have likely been minimal or absent to observers on the ground. Instead, most birds would have continued south high overhead. This event highlights the dramatic effect of precipitation on active migration.

Doug Stotz, a Senior Conservation Ecologist at Chicago’s Field Museum of Natural History shared in an email to the authors, “I have seen lots of fallouts in my 40 years in Chicago, but nothing like this. One piece was [that] the sheer number of birds was really unprecedented. The other piece is that 4 hours after sunrise thousands of nocturnal migrants were still moving rapidly south…typically within an hour or so [of sunrise] the birds settle down into the habitat and focus on foraging. That didn’t happen on Thursday.” Local long-time birders recollect only a couple other events in the same league.

One fall in the early 1970s, William Beecher of the Chicago Academy of Sciences “collected a thousand dead birds from around the base of the Hancock [Building]” located in downtown Chicago, and “lined up the carcasses in a parking lot, and invited the media to take a look” according to the Chicago Reader. We can only wonder how many birds flew over the city that day, and what conditions caused the flight, but this story rings eerily similar to the events that unfolded across the city on 5 Oct 2023.

May 1996 was also memorable for fallout in Chicago, but that event was widespread and occurred over much of the state of Illinois. Christine Williamson synthesized the event in Meadowlark (5: 129–131). Using weather information shared by local birder Paul Clyne, Williamson wrote that, “On 9 May, [a] warm front brought in a huge fallout of stranded birds. The warm front from the south banged up against the cold front just north of Chicago, dumping birds south of the collision line.” Unseasonably cold weather followed, with a recorded temperature of 34°F (1°C) on 11 May in Chicago that grounding many birds. Winds pushed many migrants over the lake, and Williamson reported that Dave Willard of the Field Museum of Natural History recounted that “one of his colleagues was walking his dog along Montrose Beach on 9 May, when he noticed thousands of tiny bodies washed up by the surf. The Field Museum staff began to collect beach kills and gathered well over 2,000 birds.”

Another fallout was documented by Paul Clyne at Burnham Park’s Promontory Point on 12 Sep 2000, when over 2700 Swainson’s Thrushes were counted.

More recently, Jacob Drucker, Lila Fried, Jing-Yi Lu, and Ryan Fuller documented a strong warbler flight of over 3,700 individuals on 6 Oct 2022, again at Promontory Point.

Even just two decades ago, there were fewer birders counting morning flights with purposeful dedication. We are grateful to have had the opportunity to document such an awe-inspiring and heartbreaking day of migration in Chicago.

A disoriented Sora attempts to find habitat on Chicago’s lakefront. 5 Oct 2023. Photo © Woody Goss.

Nathan Goldberg is a Tour Leader for Red Hill Birding. When not guiding, he serves as Secretary of the ABA’s Recording Standards and Ethics Committee and Vice President of the Illinois Ornithological Society. A Chicago native, Nathan enjoys exploring the city’s many greenspaces, as well as county listing through underbirded parts of Illinois.

Marky Mutchler is a National Science Foundation Graduate Research Fellowship Ph.D. student at the University of Chicago, where she pursues her passion of using museum, genomic, and natural history-based resources to explore diversification across the avian tree of life. As an ornithologist, illustrator, and communicator, they apply this passion to both their professional and everyday lives.

Jacob Drucker is a Ph.D. Candidate at the University of Chicago. He is interested in how birds interact with tropical climates over ecological and evolutionary time. He uses remote sensing to study migratory flyways through Colombia, and behavioral and molecular data to study the ecological plasticity of birds in the tropical Andes.

Nathan Goldberg is a Tour Leader for Red Hill Birding. When not guiding, he serves as Secretary of the ABA’s Recording Standards and Ethics Committee and Vice President of the Illinois Ornithological Society. A Chicago native, Nathan enjoys exploring the city’s many greenspaces, as well as county listing through underbirded parts of Illinois.

Marky Mutchler is a National Science Foundation Graduate Research Fellowship Ph.D. student at the University of Chicago, where she pursues her passion of using museum, genomic, and natural history-based resources to explore diversification across the avian tree of life. As an ornithologist, illustrator, and communicator, they apply this passion to both their professional and everyday lives.

Jacob Drucker is a Ph.D. Candidate at the University of Chicago. He is interested in how birds interact with tropical climates over ecological and evolutionary time. He uses remote sensing to study migratory flyways through Colombia, and behavioral and molecular data to study the ecological plasticity of birds in the tropical Andes.