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One cool, calm night in October, J. Alan Clark, a biology professor at Fordham University, sat quietly under tall sugar maples at the Mianus River Gorge Preserve, about 40 miles northeast of Manhattan. Goateed and slightly puckish, the typically chatty biologist was still, hands folded in his lap, face tilted toward a night sky streaked with wisps of clouds. Clark has a professional affinity for certain sounds that few people hear—sounds that, in fact, most people don’t even know exist. His eyes occasionally narrowing, Clark was listening, intently, his ears cocked toward the stars above.

Suddenly, a buzzy little tseep note dripped down from the night sky. Faint as a whisper, short as a quick kiss, it stood out from the trills of katydids and crickets, and Clark’s eyebrows arched over his black-rimmed glasses, a grin on his face. That was it. The birds were there. Somewhere between the treetops and the cloud bottoms, one of the least-known, albeit massive in scale, natural phenomena was under way: the nocturnal migration of birds.

On any given night in the spring and fall, hundreds of thousands—and at times millions—of birds migrate across North America. From large herons to warblers to vireos, sparrows, and other smaller species, blankets of birds flow across the continent. While it’s a largely unseen migration, it’s not a silent one. Most of these birds vocalize while on the wing, making night flight calls, dubbed NFCs by scientists, that sound nothing like the better-known melodious breeding songs of spring. Barely discernible by humans, these chirps and tweets and buzzes, flowing overhead under cover of darkness, are increasingly being captured by specialized acoustic monitoring equipment that can record, analyze, and identify the call makers.

[audioplayer:161226|align:left|caption:Blackburnian warbler] Audio description: Blackburnian warbler

In fact, on the rooftop behind Clark perched an inconspicuous microphone, connected to a wire that snaked down to a lunchbox-sized digital recorder housed in the preserve’s offices. The system was deployed by Rachel Bricklin, a Fordham Ph.D. candidate. Her analysis of night flight calls is helping tease apart the factors that stress migrating birds using urban green spaces as they “stop over” at city parks to feed and rest. It’s just one aspect of how acoustic monitoring is filling in the feathered blanks of the night. Across the country, such monitoring is allowing both research scientists and citizen scientists to ponder issues as straightforward as what is flying over a backyard feeder at night and as complex as assessing the impacts of wind turbine projects and designing bird-friendly urban landscapes. The emerging acoustic technology is also supporting massive monitoring efforts that seek to forecast bird migrations on a continental scale. “This is making the invisible visible,” says Clark. “We are now able to peek into those dark skies and quantify and clarify nocturnal migration in ways that will make the world safer for birds.”

Even the most analytically minded can’t help but be moved by such an alluring aspect of scientific inquiry—the ability to glimpse a world nearly hidden from human experience for millennia. Scientists and birders have long listened to the night sky, pondering what to make of its subtle symphonies. In the course of five hours on a September night in 1896, University of Wisconsin history instructor tallied 3,600 night flight calls from a small hill outside Madison. The count was published three years later, the first quantified record of flight-calling nocturnal migrants. The first audio recordings of nocturnal bird migration came about in the 1950s; in those days some researchers captured the calls on reel-to-reel tape recorders hooked up to large microphones set inside rings of sound-insulating hay bales. In 1987 a voice-activated system to record nocturnal migrants was developed, a huge step forward that saved both audiotape and the hours spent listening to near silence. Scientists were still stymied, however, by the daunting tasks of counting, identifying, and analyzing hundreds and sometimes thousands of recorded calls, one by one, by ear, sitting at a desk for days upon days after long nights of recording.

Recent years, however, have brought about “an explosion of professional and amateur research in acoustic monitoring and the study of nocturnal migration,” says Jeff Wells, senior scientist for the . For decades, scientists have used radar monitoring to track large-scale movements of migrating birds. Radar has limitations, however. On a finer scale, it can’t differentiate between “targets,” such as birds, bats, and large insects. It provides no information on species, and is expensive and complicated to use in the field. By contrast, microphones calibrated specifically for recording nocturnal migrants—some of the best can capture the high-frequency note of a warbler flying 1,000 feet overhead or the hoarse buzz of a grosbeak at twice that distance—have become so affordable that avid birders are adding them to their wish lists alongside high-quality binoculars. Analyzing hours of recordings remains cumbersome, but automated sound-recognition software and sound-analysis software is now available for free or for a reasonable price. “These developments,” says Wells, “have placed this field at a new threshold.”

Most birds migrate at night. The stars and the moon aid night-flying birds’ navigation. Free of daytime thermals, the atmosphere is more stable, making it easier to maintain a steady course, especially for smaller birds such as warblers that might fly as slowly as 15 miles per hour. Cooler nighttime temperatures also help keep hard-working birds from overheating. And for birds that frequently wind up on the menu of hawks, cats, and other daytime predators, flying under cover of darkness can be a lifesaver.

While scientists aren’t certain about the reasons that migrating birds call when flying at night, there are some widely accepted theories. Research has shown that birds do it more frequently in rough weather and when navigating headwinds and crosswinds, so the calls likely help birds stick with their flocks. Communicating at night might also prevent fatal crashes. High call rates coincide with incidences of mass collisions with tall, lit structures such as high-rises and offshore energy structures. It may be that night flight signals serve as a warning to other birds about such perils.

These sounds are nothing like the beloved springtime breeding choruses. “We’re talking about cryptic little buzzes and whistles,” explains Bill Evans, director of a nonprofit called , which facilitates acoustic monitoring. Evans was first intrigued with night-calling birds while delivering pizzas during college. In the 1990s he worked as a technician with Cornell University’s Laboratory of Natural Sounds, the world’s largest archive of wildlife audio and video recordings. Evans spent more than a decade recording and analyzing the night flight calls of eastern migrants. In 2002 he and naturalist Michael O’Brien published a CD with the recorded calls and spectrograms of 211 North American terrestrial bird species, an event that helped kick-start today’s revolutionary research. “Few of these calls are longer than a half-second,” he says, “and many are much shorter than that.”

[audioplayer:161236|align:left|caption:Baltimore oriole] Audio description: Baltimore oriole

Consider the hooded warbler. Its night flight call is perhaps one-twentieth of a second long. To the human ear, it sounds like a buzzy cricket-like chirp. “Analyze the spectrogram,” Evans says, “and it’s a completely different language. There’s a lot going on inside the sound.” The spectrogram reveals a cup-shaped call in the 6-to-7.5-kilohertz range, with a ragged modulation—“a smiley face with teeth,” Evans laughs. No other bird calls with that particular signature; it’s like an auditory fingerprint that reveals the warbler’s unseen presence above.

With enough practice, many night flight calls can be identified by ear. The dickcissel’s buzzy note is distinguishable from the buzzy one made by the painted bunting and the blue gros- beak. There’s the chestnut-sided warbler’s low, buzzy dzzew call. The swamp sparrow’s buzzy zee is distinct from the short whistle of the hermit thrush. Researchers have described this variety in an array of onomatopoeic notes: wok (black-crowned night heron), skwonk (great blue heron), creeeenk (least sandpiper), and vheeu (veery). Other birds are not yet identifiable by species but are grouped into “complexes” of calls—the buzzy zeet group of the cerulean, Connecticut, and magnolia warblers, for example, and the sibilant, rising tseet group of the Tennessee, Nashville, and black-throated green warblers.

[audioplayer:161251|align:left|caption:Swainson's thrush] Audio description: Swainson's thrush

It’s a confusing vocabulary, and perhaps the greatest challenge to acoustic monitoring’s potential lies in the analysis of those ephemeral, scattered sounds. A single station on the mid-Atlantic coast could capture 5,000 recorded calls during a long October night. This fall Evans is monitoring 15 stations scattered across the continent. The project might generate 30 gigabytes of data per night—the equivalent of nearly 10,000 digital photographs—which would take Evans four hours or better to analyze.

“We’re talking about a crush of data, night after night during the migrations,” says Andrew Farnsworth, a research associate at the Cornell Laboratory of Ornithology. He should know. Farnsworth led the development of the so-called “Rosetta Stone for warblers,” a groundbreaking collection of spectrograms and photographs of 48 North American warblers’ night flight calls, and he is a leader on , a joint effort to forecast bird migration. Cornell, with Oregon State University, the University of Massachusetts-Amherst, Microsoft, and the National Oceanic and Atmospheric Administration, hopes to site automated recording stations across the northeastern United States and along the Gulf Coast. “Extracting and identifying calls from thousands and thousands of hours of recordings is daunting,” Farnsworth says, “and computer scientists across the country are working on the algorithms needed to turn this data around quickly and meaningfully.”

Already acoustic monitoring is allowing researchers to ask increasingly specific questions about how migrating birds interact with the landscapes around them. Say it’s early fall, and you are a migrating scarlet tanager headed south from the boreal wilds of eastern Canada. While winging over eastern North America, the darkened earth below glitters with roadways, the twinkling of rural farmsteads, and small towns. To the east there is the onyx plain of the Atlantic Ocean. To the west is the relatively dark bulk of the Adirondacks. Then suddenly, studded with blinking lights and glittering glass towers that reach for 1,000 feet into the sky, the New York metropolitan region scrolls into view. Scientists know that migrating birds are attracted to artificially lit structures, such as communications towers and skyscrapers. And 70 percent of birds migrating to Canada from eastern North America fly over at least one urban landscape. When faced with an entire landscape shimmering like tinsel, what do birds do?

[audioplayer:161241|align:left|caption:Eastern bluebird] Audio description: Eastern bluebird

“Would you stay in the darker zones and fly to the west, tracking the Hudson River, or head east, along the East River, or go way over towards Jamaica Bay?” ponders Susan Elbin, director of conservation and science for New York City Լ����Ƶ. At her office 15 floors above 23rd Street, Elbin, face ruddy from a recent trip where she was banding songbirds on nearby Prall’s Island, shares with migrating birds an altitude at which they are pre- sented with such difficult choices. “Would you just power on and try to gain altitude over all of Manhattan’s high-rises? Or would you careen through the skyscrapers? We don’t know how birds navigate these massively developed landscapes, and we need to know, because so much of the country is only going to become increasingly urbanized. Acoustic monitoring, coupled with radar, is the one tool that might finally give us some answers.”

Bricklin, the Fordham graduate student, is working to tease apart those questions. She monitors bird activity in two research sites—the suburban and the , where her recording equipment is deployed on the World of Birds exhibit building. In the Bronx, birds seeking to refuel during migration are exposed to all the stressful elements that bedevil human city dwellers—from sirens and helicopters to car horns and crowds—plus dogs, cats, bright lights, and more. By capturing and testing birds for blood levels of corticosterone, a hormone released during stressful situations, Bricklin is looking for clues into how birds stopping in city parks respond to stress. The first step, she explains, “is to figure out the differences between which species are stopping at each site and which are flying over. That would be impossible without these new acoustic monitoring methods.”

Elsewhere researchers are studying migrating birds in less urbanized areas. In the North Sea off the coast of Germany, which migrating birds cross as they pass from Scandinavia through the Netherlands, German researchers recorded the calls of Eurasian blackbirds, redwings, song thrushes, and about a hundred other bird species to study what weather conditions prompt migrating birds to congregate at illuminated offshore wind turbines and other structures. Using specialized microphones shielded from gull poop by a stainless steel cover, the biologists ran the data files through software that filtered out the background noises of wind, waves, and rain. The study showed that night-migrating birds called more frequently during nights with high collision rates. In the future, these scientists say, acoustic monitoring systems could serve as a real-time automated collision risk alarm system.

[audioplayer:161246|align:left|caption:Black-billed cuckoo] Audio description: Black-billed cuckoo

That was one of the goals of an emergency monitoring effort recently completed in Pennsylvania. Last September Լ����Ƶ Pennsylvania, New York City Լ����Ƶ, and the American Bird Conservancy learned of a massive public art display planned for Philadelphia that involved 24 robotic searchlights sweeping the night sky for 25 nights during the peak of fall migration. Working frantically in the days before the project’s opening, conservationists convinced the architects to make design changes and to add an emergency alert system in case the lights appeared to confuse migrants. Clark and other researchers assembled an avian radar detection system and three acoustic recorders to monitor bird movements during the light show. Over the course of the project, more than 735,000 migrating birds were tracked through a one-kilometer-wide (about six- tenths of a mile) radar transect over the light show. Thankfully, the monitoring showed no significant problems for the birds, but the studies turned up some intriguing facts. The acoustic monitoring devices recorded six times the number of night flight calls in the vicinity of the light show than at a recording station at the East Reservoir, less than two miles away.

Those findings are right in line with another of Clark’s research projects, a study recording migrating birds’ night flight calls as they passed over North America’s largest metropolis. During just 10 full nights and three partial nights of study in May 2010, he and his colleagues counted more than 300,000 “targets” (some were bats and large insects, but the vast majority were birds) flying over two roughly 550-yard-wide transects of New York City. “And that’s a huge underestimate,” Clark says, because many birds fly higher than the effective range of microphones.

It’s not only career scientists and conservationists using acoustic monitoring to study and assess bird migrations. In fall 2011 and spring 2012, Jessica Fleischman, then an 11th grader in Ossining, New York, set up low-cost microphones on the roof of Ossining High School. She compared her data to sounds recorded on a rooftop in rural Alfred, New York, 263 miles to the west. In 84 nights of recording, she tallied more than 20,500 nocturnal flight calls. With assistance from Clark and Evans, Fleischman showed that wind speed and precipitation were critical factors in calling rates. Her work won her a finalist spot in the American Museum of Natural History’s prestigious and a Gold Medal placement (top 10 percent of finalists) at the 2013 .

As scientists learn more about how birds navigate through cities, the data could result in designing urban spaces in ways that mitigate danger. During spring and fall 2012, New York City Լ����Ƶ deployed acoustic monitoring units similar to Bricklin’s devices on four buildings along a transect cutting through Manhattan—including the Bank of America Tower at Bryant Park—at heights above ground level from 40 feet to 945 feet. By dissecting who is flying where, and what influences migratory routes through and over cities, scientists could devise detailed “risk maps” of urban landscapes, identifying high-threat and low-threat zones. You could have the world’s worst building design in terms of bird collisions, Elbin explains, but if it’s not in a place where there are migrating birds, it’s not so problematic.

[audioplayer:161256|align:left|caption:Hairy woodpecker] Audio description: Hairy woodpecker

“This is very exciting,” she says. “Acoustic monitoring is such a rich vein to mine. For years we’ve been picking dead birds off the streets, and it’s so sad. But this work shows that even though there are problems, the vast majority of birds migrate successfully. And there is more we can do to help.”

A few weeks after his nocturnal peregrinations, Clark received an analysis of what the microphones caught on that dark October New York night. Bricklin used two Evans-developed software programs—Tseep-x and Thrush-x—to analyze the Mianus River Gorge Preserve data. The algorithms parsed out the calls of migrating birds from the trills of insects, distant door slams, and the shuffling noises Clark made as he shifted around in the cool air.

Farnsworth, who assisted with the analysis, identified 26 nocturnal flight calls recorded over the course of the evening, a relatively light night for migrating birds. Among the birds he heard: black-throated blue warbler, chipping sparrow, savannah sparrow, more tseep-group birds, and another dozen calls of uncertain origin.

[audioplayer:161231|align:left|caption:Chipping sparrow] Audio description: Chipping sparrow

At precisely 8:55 p.m., the microphone picked up the sound Clark heard: a super-short, buzzy zeet. Part of the tseep group of nocturnal flight calls, it’s a note made by a half-dozen migrants, such as blackpoll, cerulean, and worm-eating warblers. The call lasted an eyeblink-brief 0.06 seconds, dipping down to 6.26 kilohertz, a range well within what a human can discern. On a spectrogram, the call looks like a tiny wave train, double dark bands with sharp peaks produced by its buzzy tones. It now resides in Bricklin’s research, a permanent portrait of an ephemeral moment in the night.