Tuning in to Invisible Migrations

Researchers harvest nocturnal flight calls to aid in bird conservation

By Pat Leonard

Each spring and fall, living rivers flow above our heads in the dark of night, powered by instinct and rushing wings as millions of birds fly overhead. Although it is too dark to see these nocturnal migrants, researchers are learning a lot about them by tuning in to fleeting calls that last just a fraction of a second and are almost too high for humans to hear.

Researchers can determine which migrants are passing overhead by using software to match recordings of night flight calls with "voice-prints" for each species. Above, a spectrogram showing the Blackburnian Warbler's high-pitched tzzd call—listen. Recordings from William R. Evans and Michael O'Brien, Flight Calls of Migratory Birds Photos by Evan Barbour (3)

Scientists at the Cornell Lab of Ornithology are studying these nocturnal flight calls to help the Department of Defense monitor birds over its military bases. One goal is to demonstrate that researchers can quantify the numbers of birds passing overhead and identify the species from recordings of night flight sounds, work pioneered in the mid-1980s by former Lab associate Bill Evans. The study could lead to a continentwide monitoring system that helps answer questions about bird migration and the health of the environment.

A spectrogram shows the Ovenbird's piercing seet call used in nocturnal migration—listen.

So far, researchers have collected data from migration during fall 2005 and spring 2006 at an array of sites stretching from Fort Drum in New York to the Patuxent River Naval Air Station in Maryland. "We're excited to analyze the data from last October, because they include one of the largest fallouts of migrants ever witnessed in the Northeast," said Ken Rosenberg, the study's principal investigator and the Lab's director of Conservation Science.

The project uses autonomous recording units (ARUs) created by the Lab's Bioacoustics Research Program to collect sounds, along with MP3 units. The Lab's programmers have developed computer software called XBAT to recognize and filter out the night flight calls of particular bird species from thousands of hours of recordings.

Studies by Cornell University graduate student Andrew Farnsworth show that warblers have distinctive night flight calls, even though calls of the same species, and even of the same bird, may vary. XBAT will create a "voice-print" for each species that will enable scientists to tally the number of individuals passing overhead each night.

The researchers must also find new ways to deal with a mountain of data. Analysts must run tens of thousands of hours of ARU recordings through XBAT, then human eyes and ears are needed to sift through the results. The long-term goal is to fully automate this process.

The Common Yellowthroat's nocturnal flight call is shown in the spectrogram—listen.

Over time, night flight migration data may help reveal whether certain bird populations are declining. Some birds breed and winter in areas that are difficult to monitor, such as the sparsely-populated boreal forests of Canada or Central and South America. The only chance that researchers have to monitor some of these species may be during migration. Conservationists could also use the acoustic techniques to measure where and at what height birds are flying for environmental assessments on placement of wind turbines.

Each year, we see only a small portion of the massive river of birds flowing overhead. "For some birds, like the Gray-cheeked Thrush, you might see one or two during the fall or spring migrations, but you can hear more of them in one night than you'll see in a lifetime of bird watching," said Rosenberg. "Hundreds and hundreds of them are flying over and most people are unaware of it." Acoustic night-flight projects like this one complement other traditional methods of bird monitoring to present a more complete and accurate picture of the mystery unfolding, note by note, in the skies above.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu