Ornithologist Bill Evans has been tracking down avian night flight calls for 17 years.
Many North American birds are in serious decline. But scientists aren’t sure what’s wrong because birds are hard to count. The problem is partly that birds often migrate long distances between wintering sites and summer breeding grounds. Usually, they fly unobserved at night, and in many cases scientists don’t know what route they take. However, a new technique promises to solve this problem. The Great Lakes Radio Consortium’s Daniel Grossman has our story:
Transcript
Many North American birds are in serious decline. But scientists aren’t sure what’s wrong because birds are hard to count. The problem is partly that birds often migrate long distances between wintering sites and summer breeding grounds. Usually, they fly unobserved at night, and in many cases scientists don’t know what route they take. However, a new technique promises to solve this problem. The Great Lakes Radio Consortium’s Daniel Grossman has our story:
(Sound on dock, with Evan’s whispering out bird names fades up under intro.
Continues under Grossman track, then heard in clear.)
It’s a warm spring evening on the south Texas coast. Ornithologist Bill Evans sits on a dock, listening for birds.
“Grey-cheek thrush.”
Grossman: “That high one?”
“Yeah. (makes sound). Moor hen calling behind us on the ground. Oh sorry, that’s a black-necked stilt.”
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 what to do with his life.
“I was getting back to a campsite about two in the morning and heard an incredible flight.”
Hundreds of birds were passing overhead in nocturnal migration, including what appeared to be about 100 black-billed cuckoos.
“If you go out and look for black-billed cuckoos during the day you may see two or three. And I’m thinking, ‘wow if I had a tape recorder and could somehow document this on audiotape, I might have a pretty powerful conservation document.'”
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 Walcott, the calls they make during night flights are another matter.
“If you out on an evening and listen to these birds migrating overhead you’ll hear all these twitters, most of which don’t sound anything like what a normal bird sounds like at all.”
Walcott 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.
“And to be able to recognize individual species by their calls was a dream that many people have had. And Bill is the first one that’s been able to do it on any substantial scale.”
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.
“The herons. Amazing squawks. Black-crowned-night heron is a sort of (makes sound). Green heron is a (makes sound). Barn owl. It’s a [makes sound]. Except it’s about ten times louder than that. The dickcissel is actually a sparrow and it’s got sort of a buzzy note (makes sound).
(Sound of truck door closing and truck starting up)
The small, colorful dickcissel is why Bill Evans is here, just north of Brownsville, Texas. He’s set up a network of 15 computerized monitoring stations that listen for dickcissels 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.
(Sound of high school PA System making announcement cross fades with truck under previous track. Also mixed in sound of crowded high school corridor with students changing classes. Cross-fades with sound of classroom.)
Evans: “Come on over guys. My name is Bill, this is Dan.”
Peter: “Bill and Dan?”
Evans: “What’s your name?”
Tate: “Tate.”
Peter: “Peter.”
Bill: “Nice to meet you guys.”
Some curious students pay Evans a visit at La Ferria High School.
(Evans fades up underneath Grossman. Then heard in clear.)
Evans (to students): “…So anyway the sound comes down this audio cable into this computer. We’re just checking the data from last night…”
(Computer keyboard sounds in actuality fades under track.)
The computer has a program that distinguishes the call of the dickcissel from other birdcalls and extraneous noise. The machine records the call, and saves a picture – or spectrogram – of it. Evans trouble shoots his stations and collects data daily. First he winnows out false positives, sounds that tricked the computer, by inspecting the spectrograms.
Evans (to students): “I’m going to set up one folder to put in the dickcissels’ 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. We had 28 here that we classified as dickcissels…”
After checking the computer and collecting its data 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 dickcissel migration, bringing a huge flight of birds.
Evans (to students): “…And this weekend we think there are thousands of them just in Northeast Mexico. They’re going to take off. ‘Cause last year in one night, we had over 3,000 detected at McAllen High School…”
In the end, the big flock didn’t appear until the following week. Though the arrival was delayed for 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.
(Sound of truck door closing and truck starting up.)
Walcott: “It’s really an extraordinary accomplishment.”
Cornell professor Charles Walcott 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 Walcott, has neither a college nor any other degree.
Walcott: “With Bill’s scheme you can now say, well, this was an evening when we had a huge migration of warblers and they were of the following species. 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.”
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. This spring, in an important first step, Evans and collaborator Michael O’Brien released a compact disc with night flight calls of most eastern land birds. Now anyone can learn the secrets Bill Evans has unlocked.
For the Great Lakes Radio Consortium, this is Daniel Grossman.