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Public Radio's Environmental News Magazine (follow us on Google News)

Nocturnal Birds

Air Date: Week of

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Migratory birds often travel at night, making it difficult for scientists to monitor these declining flocks. But one researcher figured out how to track these birds by using their distinctive nighttime vocalizations.


TOOMEY: Much of what goes on in the animal world remains a mystery to us, simply because some things are difficult to observe. For instance, many North American migratory birds are in serious decline but scientists aren’t sure why.

And since a good portion of the journey they make to their summer breeding grounds is done at night, researchers don’t often know what routes these birds take. But a new technique promises to answer that question. The Great Lakes Radio Consortium’s Daniel Grossman reports.

EVANS: It’s a little high warbler call right here.

GROSSMAN: It’s a warm spring evening on the south Texas coast. Ornithologist Bill Evans sits on a dock, listening for birds.

EVANS: Gray-cheek thrush.

GROSSMAN: That was that high one?

EVANS: Yeah. [WHISTLE] Moorhen calling behind us on the ground. Oh no, sorry. That’s black-neck stilt.

GROSSMAN: Evans has been listening to and studying these night calls since he had an epiphany at a Minnesota campground in 1985. Then, a recent college dropout and avid birder, Evans was adrift, unsure of what to do with his life.

EVANS: I was getting back to a campsite about two in the morning and heard an incredible flight.

GROSSMAN: Hundreds of birds were passing overhead in nocturnal migration including, what appeared to be, about 100 black-billed cuckoos.

EVANS: If you go out and look for black-billed cuckoos during the day, you may only see two or three. And, I’m thinking that, wow, if I had a tape recorder that could somehow document this on audiotape, I might have a pretty powerful conservation document.

GROSSMAN: Bill Evans got a recorder and soon was making tapes of flight calls. But his recordings were of limited use because no one knew which birds made which sounds. The melodious tunes birds perform during the day are well-known. But according to Cornell professor Charles Wolcott, the calls they make during night flights are another matter.

WOLCOTT: If you go out on an evening and listen to these birds migrating overhead, you hear all sorts of little twitters, most of which don’t sound anything like what a normal bird sounds like at all.

GROSSMAN: Wolcott is the former director of the Cornell Ornithology Laboratory. He says night calls may help birds keep from colliding with each other. For decades, researchers hearing these calls were frustrated, knowing there were birds in flight but unable to determine which ones.

WOLCOTT: And to be able to recognize individual species by their calls was a dream that many people have had. And Bill is really the first one that’s been able to do it at any substantial kind of scale.

GROSSMAN: Evans spent the next 17 years prowling migration routes to match birds with their calls. Often, his only chance came in the wee morning hours when sometimes night migrants make a single night call before settling down to eat and rest. Gradually, he cracked the code.

EVANS: The herons’ amazing squawks, black-crowned night heron is sort of a COOCK. Green heron which is a SKE-OW. Barn owl, it’s a PSSSH, except it’s about ten times louder than that. The dicksissel is actually a sparrow and it’s got sort of a buzzy note that’s more of a PHRBT.

GROSSMAN: The small, colorful dicksissel is why Bill Evans is here, just north of Brownsville, Texas. He set up a network of 15 computerized monitoring stations that listen for dicksissels in flight. It’s the first large-scale effort to track birds using night calls. The network is stretched out along a line he believes these birds cross on their way between Venezuela and the U.S. plains.

Each station has a roof-mounted microphone connected to a computer. Most of them are at high schools. Their large flat roofs and spacious grounds reduce traffic noise. And they’re generally in a science class. And while Evans does stay up late to listen for pleasure, it’s these computers that are actually doing the work. Each day, he collects the past night’s results in a marathon drive, station by station.

EVANS: Come on over, guys. My name is Bill. And this is Dan.

STUDENT: Bill and Dan.

EVANS: What’s your first name?


GROSSMAN: Some curious students pay Evans a visit at La Feria High School.

EVANS: So anyway, the sound comes down this audio cable into this computer. And, we’re just checking the data from last night.

GROSSMAN: The computer has a program that distinguishes the call of the dicksissel from other bird calls and extraneous noise. The machine records the call and saves a picture or spectrogram of it. Evans is here to check for problems and to collect data. First, he winnows out false positives, sounds that trick the computer by inspecting the spectrograms.

EVANS: I’m going to set up one folder to put in the dicksissel calls, and the other, I’m going to put in the noise, the false detections. So now, I’ve just classified the detections from last night. And I can go to the folder. We had 28 here that we classified as dicksissels.

GROSSMAN: His job here over, the researcher says he has to run. Each of the 15 stations in his network needs a checkup because this weekend might be the climax of the dicksissel migration, bringing a huge flight of birds.

EVANS: And this weekend, we think there’s thousands of them just in northeast Mexico. They’re going to take off and fly over the valley. Because last year, on one night, on April 28th, we had over 3,000 detected at McAllen High School in one...

GROSSMAN: In the end, the big flock didn’t appear until the following week. Though the arrival was delayed by several days compared to the previous year, Evans now has proof that, by monitoring night calls, he can predict the timing and migration route of an individual bird species.

WOLCOTT: It’s really an extraordinary accomplishment.

GROSSMAN: Cornell professor Charles Wolcott says the migration information Evans is discovering can’t be collected any other way. It’s all the more extraordinary because Bill Evans, who once worked for Wolcott, has neither a college nor any other degree.

WOLCOTT: And this is very useful and very interesting information. And it gives you a sense of where the migratory paths for each species of birds might be.

GROSSMAN: It’s detailed information like this that conservation specialists need to design plans to protect the most threatened species. In the future, Evans hopes several large-scale computer networks of the sort he’s testing in Texas will monitor many species throughout the United States. He hopes the listening posts could help solve the mystery of why so many North American species are in decline. For Living on Earth, this is Daniel Grossman on the Gulf Coast of Texas.




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