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Catching Fall Migrants by Ear

Step outside on a clear, cool night this fall, ideally one with a light breeze out of the north, and stand still for a few minutes. Let the crickets fade into the background. If you’re patient, you may hear it: a thin, high seet from somewhere overhead, then a soft, liquid peep, then nothing for a while, then another. You can’t see who made them. You never will. But those faint notes are the voices of songbirds passing over your roof in the dark, part of one of the largest animal movements on Earth.

It happens every fall, right over our heads, and most people never notice. The birds leave the woods at dusk, climb into the night sky, and fly for hours, a warbler weighing about as much as two quarters heading for Central or South America. By sunrise they drop into whatever patch of trees they can find, refuel, and wait for the next good night.

This fall we’ve been listening in. Using a microphone pointed at the sky here in Murrysville and BirdNET-Cloud’s Nighthawk flight-call detector, we’ve been recording and identifying these nighttime travelers as they pass over. Here’s what night migration is, why it happens the way it does, and what our little station has been hearing.

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The Great Migration You’ve Never Seen

Most of North America’s migrating songbirds travel at night. Warblers, thrushes, sparrows, tanagers, grosbeaks, and cuckoos all do most of their long-distance flying after dark. On the busiest fall nights, more than a million birds can pass over the Pittsburgh region in a single night, according to the National Aviary. Across the whole continent, peak nights put hundreds of millions of birds in the air at once.

The best way to picture it is on weather radar. On a big migration night, the radar around Pittsburgh “blooms” shortly after sunset: a doughnut of blue and green spreading out from the radar site, just like light rain. But it isn’t rain. It’s birds, tens of thousands of them, rising into the sky within the same hour. Cornell Lab of Ornithology’s BirdCast project turns those radar returns into nightly counts and forecasts, and you can look up how many birds crossed Westmoreland County last night on their dashboard.

Here in western Pennsylvania, fall migration builds through late August, peaks from early September through October, and tapers off into November as the later sparrows and kinglets come through. Most of these birds fly somewhere between a few hundred and a few thousand feet up, much too high and too dark to see, but not too high to hear.

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Why Fly in the Dark?

At first it seems backwards. Songbirds are daytime animals; they feed, sing, and defend territories by daylight. So why do they save their longest flights for the night?

The night offers several advantages at once. The air is cooler and calmer after sunset, with less of the churning turbulence that daytime heating creates, so a small bird flapping for hours loses less water and burns less energy. The hawks and falcons that hunt songbirds by day are mostly grounded. And the night sky itself is a navigation aid: songbirds read the stars and the fading light at sunset to set their compass, alongside a remarkable ability to sense Earth’s magnetic field.

Flying at night also frees up the daylight for what migrants need most, which is food. Some warblers nearly double their body weight in fat before a long leg of the trip, and they build that fuel by foraging all day in the same trees where they landed at dawn. That’s why a patch of woods that was quiet on Tuesday can be full of warblers on Wednesday morning: they arrived overnight.

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Reading the Sky: Weather and the Big Nights

Anyone who follows BirdCast notices that migration doesn’t flow steadily. It comes in pulses. A couple of quiet nights, then suddenly a huge one. I’ve been tracking the weather against those big nights for a while, especially barometric pressure, expecting the birds to key on a falling or rising barometer. I never found a clean correlation, and it turns out there’s a good reason.

In fall, the birds want a tailwind, and the best tailwinds come behind a cold front. When a front sweeps through and the wind swings around to the northwest, the air turns cool and clear and conditions become ideal for flying south. Pressure does tend to rise behind a front, but pressure is only a stand-in for what the birds actually care about: wind direction and temperature. BirdCast’s forecast models, which predict migration intensity surprisingly well, lean far more on temperature, wind, and the date than on pressure alone.

Two more things muddy the picture. First, the birds flying over Murrysville at 2 a.m. didn’t take off from Murrysville. They likely took off at dusk a hundred miles or more to the north, so it’s the weather there that triggered their decision. Second, birds pile up. A string of rainy nights or south winds grounds migrants by the thousands, all fattened up and restless. When the first good night finally arrives, they all leave together, and that’s how you get a record-setting night that looks like it came out of nowhere. Researchers studying radar data in Europe found the biggest migration nights could only be predicted using weather from the previous three days, not just that night’s conditions.

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Tiny Voices in the Night: What Flight Calls Are

While they fly, many of these migrants call. These nocturnal flight calls are nothing like the songs you hear in May. They’re short, simple notes, typically lasting somewhere between a twentieth and a third of a second, pitched anywhere from about 1 to 9 kHz. Many warblers and sparrows give thin, high zeeps and seets near the upper edge of human hearing. Thrushes are lower and richer: a Swainson’s Thrush gives a liquid peep often compared to a water drop, and the Veery a descending veer.

Why they call is still not fully settled. The leading ideas are that flight calls help birds keep in contact with one another in the dark, hold loose groups together, and possibly help with decisions like when to land. A 2025 study analyzed 18,300 hours of flight-call recordings from 26 sites across eastern North America and found something remarkable: different species travel together at night more often than chance would predict, especially species with similar wing lengths that fly at similar speeds. The night sky, it seems, is more social than we thought.

People have been listening for these calls since the late 1800s, when early ornithologists stood outside at night and counted thrushes by ear. Modern flight-call recording owes a great deal to pioneers like Bill Evans of Old Bird, whose microphone designs made it possible to capture these faint calls reliably. His Old Bird 21c flight-call microphone is the one on top of our station right now.

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Listening In: BirdNET-Cloud and Nighthawk

Radar is wonderful at telling us how many birds are flying and in what direction, but it can’t tell a thrush from a warbler. A microphone can. That’s what makes flight-call monitoring such a good complement to radar: the radar tells us how many are up there, and the microphone tells us who.

Our station is simple: a Raspberry Pi computer, the Old Bird 21c microphone aimed straight up, and BirdNET-Cloud software listening all night long. When a call comes through, it’s analyzed by Nighthawk, a machine-learning model developed by Benjamin Van Doren and colleagues specifically for nocturnal flight calls. Nighthawk was trained on recordings from 82 species, 18 families, and 4 orders of birds that call during night migration, with its strongest coverage in eastern North America, which is exactly where we are.

One thing I appreciate about Nighthawk is that it’s honest about what it doesn’t know. Many warbler flight calls are so similar that even experts argue over them. When a call can’t be pinned to one species with confidence, Nighthawk reports a broader group instead, such as “New World Warblers” or “Thrushes,” rather than guess. Each detection lands in BirdNET-Cloud with its time, a short audio clip, and a spectrogram (a picture of the sound), so you can see and hear exactly what the microphone caught.

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What We’ve Heard So Far This Fall

Since the station went live on August 1, 2026, it has logged 3676 flight-call detections over 41 nights. The busiest night so far was September 6th, with 436 calls. That night lined up with northwest winds behind a cold front / a BirdCast estimate of 4 milloon birds crossing Westmoreland County, a textbook example of what the birds wait for.

Among the species and groups our microphone has picked up: Veery, Scarlet Tanager, American Redstart, White-throated Sparrow, and a steady stream of calls identified as New World Warblers, Thrushes, and New World Sparrows. Warblers have been the backbone of the count, with 1472 detections, which fits: September is peak warbler season, and they’re some of the most vocal night fliers we have.

We’re also honest about the misses. A microphone pointed at the sky hears everything: passing cars, distant trains, fireworks, and the occasional sound the model mistakes for a bird. Nighthawk even includes categories for noises like engines and fireworks so it can set them aside. We review spectrograms of unusual detections before counting them, because a machine-learning model is a very good first pass, not the final word.

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The Open Question: Do They Decide Together?

Watching the radar all season, I kept coming back to one question: how do so many birds, spread across hundreds of miles, all seem to decide on the same night? It almost looks coordinated.

The science so far says weather does most of the work. Thousands of birds reading the same wind, the same temperature drop, and the same sunset will make the same choice without needing to talk it over, the way everyone on a street opens an umbrella when the rain starts. Most birds leave within an hour or so after sunset, which is why the radar blooms look like a single synchronized liftoff.

But that doesn’t mean the calls are meaningless. We now know species travel together at night, and flight calls are the likely glue holding those groups together. Whether calls play any part in the decision to go, beyond keeping company once aloft, is still an open question. That’s part of why stations like ours matter: every night of recordings, lined up against weather and radar, adds a small piece to a big puzzle.

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How You Can Help, and How You Can Listen

The biggest threat these night travelers face near our homes is light. Bright lights pull migrating birds off course, and many end up circling lit buildings until they’re exhausted or hit glass. Interestingly, a 2019 study of more than 70,000 collisions in Chicago and Cleveland found that species which give flight calls were the ones most likely to hit lit buildings, possibly because disoriented birds call and draw others toward the light with them. The fix is easy: the National Aviary’s Lights Out Pittsburgh program asks residents to turn off unnecessary outdoor lighting from midnight to 6 a.m. during fall migration, September 1 through November 15. Most collisions happen at ordinary homes and low buildings, not skyscrapers, so your porch light counts.

If you’d like to follow along, check the BirdCast migration dashboard for Westmoreland County on any fall evening to see how many birds are expected overhead. Then go outside around 10 or 11 p.m. on a big night, get away from traffic noise, and listen. Once you hear your first seet from the dark, you’ll never think of a quiet fall night the same way again.

And if you want to go further, you can build your own flight-call station. BirdNET-Cloud runs on an inexpensive Raspberry Pi, and a Nighthawk station is set up specifically to listen for night migrants. You can find everything at birdnetcloud.com, see our live migration station at pixcams.com/bird-migration-live, and share your builds, spectrograms, and questions in the BirdNET-Cloud forum at pixcams.com/discussion-group. The more ears we have pointed at the sky, the more we’ll learn about the millions passing overhead, unseen.