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.
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 Audubon 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.
“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.
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.