Listening to Migrating Birds at Night May Help Ensure Their Safety
On autumn and spring evenings, hundreds of thousands, sometimes millions of birds migrate across North America. Cutting-edge recording devices are capturing the tiny chips and chirps these birds make while in flight, helping conservationists plot a protected course.
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.
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 Orin Libby 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.