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Flying above us while we sleep, nocturnal migrants push the boundaries of what we know about birds. Each fall and spring, millions of migratory birds travel by night between their southern overwintering grounds and their northern breeding grounds for the summer. While researchers have made significant strides to better understand this biannual phenomenon, night migration remains steeped in mystery due to nocturnal migrants being difficult to count and identify.

Fortunately, the birds themselves offer a clue: They make species-specific flight calls that scientists and birders have learned to record, identify, and count. Once limited to researchers and only the most hardcore of the birding community, now, a new wave of birders is tuning into these calls, using DIY recording equipment to experience what some cheekily call “the dark side of birding.” 

Nocturnal flight call (NFC) recording is a different kind of birding. It doesn’t require binoculars or even stepping foot outside. Instead, a microphone placed on a roof or wedged into an apartment window allows birders to eavesdrop on migrating birds overhead. On calm, quiet nights, inexpensive microphones are able to record birds calling hundreds of feet in the sky—far beyond the reach of our hearing—and cache their calls on a recording device for later review and identification. 

Across the globe, birding’s popularity surged during the COVID-19 pandemic, and for many birders, NFC recording offered an innovative way to bird while remaining safe at home. In the United Kingdom, where NFC recording is known as "Nocmig," Mark Pearson began recording nocturnal migrants just as the pandemic—and spring migration—were hitting in full. Pearson, a lifelong birder, writer, and nature guide, has watched the trend ignite throughout the U.K. in the past year. “Nocmig has gone absolutely through the roof, from a niche of bird study to wonderfully popular and populist, and anybody can do it,” says Pearson, who has recorded birds that he rarely or never sees birding during the day. “Anything can fly over. It's magic.”

While some NFC enthusiasts like to listen live on peak migration nights, most record while they sleep. Nocturnal flight calls are typically less than a second long and notoriously challenging to identify by ear, so recording them for later review makes the ID process easier. After a night of recording, birders use free computer software like Audacity to transform the night’s bird activity into a spectrogram, or picture of sound, and that’s when the excitement really starts. The tseeps, puwi, and bzeew sounds that birds voice at night become squiggles, swoops, and slides on a spectrogram—distinct visual signatures that help pin down the bird’s identity. And because what flies over at night doesn’t depend on the habitat below, the possibilities of what you can detect are endless.

For newcomers to this burgeoning type of birding, inexpensive recording devices and a growing set of resources are making it more accessible than ever. A  created in 2009 has become a hub for the growing NFC community in the U.S. Uncertain about an identification? Upload your recording and it’s sure to spark a conversation between experts and beginners alike. Corey Husic, a Los Angeles-based NFC enthusiast and the group’s founder, says Facebook was the perfect place for the then-nascent community to gather. “We’ve gone a long way over those years. Most discussions back in 2009 were reports from people listening, now you’ll find discussions about programming and software as some people turn towards automation for recording, detection, and identification,” said Husic in an email, adding that the group has doubled membership to more than 2,000 since the pandemic began.

While NFC recording is a way for many birders to step up their birding and boost yard lists, their observations are also feeding important community science databases. , an online library of bird sounds for more than 10,000 species, has a designated NFC category to submit and search for recordings. Meanwhile, the Cornell Lab of Ornithology’s eBird, the largest community science birding platform, now has a  for sharing NFC checklists. Benjamin Van Doren, a postdoctoral associate at Cornell, says the number of NFC checklists submitted in the U.S. doubled during the first year of the pandemic, with no sign of slowing down in 2021. “I think we’re at the cusp of being able to spread nocturnal recording and monitoring of bird migration really far and wide, and start to use it for real, meaningful monitoring,” Van Doren says.

In what has been called a “golden age of observation,” scientists have been employing a host of technologies to unravel the mysteries of nocturnal migration in recent decades. Radar has emerged as an especially powerful tool to study the timing and magnitude of how birds fly by night, but with one big caveat—it can’t identify species. In that way, NFC recording is unique, helping prove that Pine Siskins call and migrate at night, for instance, and offering a deeper look into species-specific migratory behavior. More so than tracking devices and radar, NFC birding also runs the gamut in terms of technological sophistication, from listening to calls by ear and using simple recording devices like a smartphone to more complex recording setups and automatic call-detecting software.

Bill Evans remembers a time when few nocturnal recordings existed. After a life-changing experience listening to a massive night-flight as a college student in 1984, Evans dedicated his life to studying nocturnal calls and making NFC accessible to others. In 2002, he completed the first-ever NFC  with Michael O’Brien, a foundational resource for learning vocalizations. “You don’t have to go to Costa Rica, you don’t have to go to Alaska to bird, you can tune in to a lot of these species going right over your house,” Evans says. 

And for some species, like the shy Gray-cheeked Thrush, an elusive nocturnal migrant that passes through the U.S. between its overwintering sites in northern South America and its breeding grounds in northern Canada, NFC recording holds an edge over birdwatching. On a single, good night, says Evans, it’s possible to record more than a hundred individuals—more than one is likely to see in a year of daytime birding anywhere in the country.

As the DIY recording community thrives, scientists are working to unleash NFC's full potential. Efforts like and , for example, are busy training neural networks to make identification of even the trickiest nocturnal calls possible with the touch of an app, and to handle the massive amounts of data generated by large arrays of microphones. In Montana, MPG Ranch has already gathered more than  20 years worth of recordings using one such array, with the hope of understanding nocturnal migration across the West’s complicated mosaic of private and public lands. And in California, an upcoming project plans to deploy more than 2,000 microphones along the Sierra Nevada mountain range that will collect roughly 119 years of acoustic data this summer alone. 

These endeavors—along with those of individual birders at home—should help researchers soon gain an even clearer understanding of nocturnal migration. And in the meantime, for DIYers, being part of the bustling NFC community, where experienced recordists eagerly welcome and help newcomers, is its own reward. “That’s where the fun is,” Evans says. “Whatever age someone gets involved.”