The Technology Behind the Recordings

Bioacoustics technology aids ivory-bill search

By Roger Segelken
April 28, 2005

If a clear, time-stamped photograph is irrefutable evidence that the Ivory-billed Woodpecker lives, then recorded sounds of Campephilus principalis--and not something else that sounds almost like it--are high-tech "breadcrumbs."

Russ Charif, a biologist in the Cornell Laboratory of Ornithology's Bioacoustics Research Program, the source of some of the world's most advanced natural-sounds recording and analysis tools, explains: "Think of these recorded sounds--the signature double-rap of the ivory-bill or its 'kent' call--as bread crumbs leading a camouflaged photographer to the base of the tree for that once-in-a-lifetime photo."

Across the room in the bioacoustics lab, earphone-clad analysts watch intently as sound spectrograms--visual representations of sounds--scroll across computer monitors. The analysts pause every few seconds to listen to an "event" that the sound-analysis software has highlighted from thousands of hours of recordings.

Is that really the "kent" of the Ivory-billed Woodpecker? Or is it just a nuthatch, or even a Snow Goose, with a similar vocalization? Back in 1831, the ivory-bill's nasal sound reminded John James Audubon of "a high, false note on a clarinet." One 21st century birder said an ivory-bill sounds "like a nuthatch on steroids."

Play it again: Was that the ivory-bill's "BAM-bam" double knock? Nine other Campephilus species rap twice on wood to make their long-distance communication signal, but those woodpeckers all live from Mexico south to South America. Is this the "BAM-bam" that will finally lead searchers to North America's most elusive bird? Or just another gun shot?

Whatever the source, the sounds must first be recorded:

  • The sound recording-and-analysis process begins in the field with devices called ARUs, for "autonomous recording units." A compact block of electronics gear about the size of a palm-held computer includes a small hard-drive and the circuitry to start and stop recording--typically about four hours when ivory-bills wake up in the morning and four hours before they retire in the evening--plus a processor for digital signals. The gear is concealed in a two-foot-long piece of PVC plumbing pipe.
  • Outside the PVC pipe, a furry wind sock covers the 16-microphone array to filter some ambient noise, although plenty of non-bird sounds--distant train whistles and too-close-for-comfort gunshots, for example--become part of the recording.
  • Two 6-volt lantern batteries power the unit for each week-long recording session. Then a technician returns by canoe to retrieve the ARU and download the recording to a larger hard drive, which is shipped back to the lab in Ithaca, New York, for analysis. Equipped with fresh batteries, the ARU is then moved to another location.
  • The ARUs are strapped to trees--safely above water level, it is hoped, although periodic flooding can change water levels in the search area 10 feet or more in a month.
  • To survey the 160,000-acre search site in the White River National Wildlife Refuge in Arkansas, ARUs are moved frequently and spaced as far apart as possible. The bird's "kent" call can be recorded up to 200 meters (656 feet) away, and the double-rap sound carries about 300 meters (984 feet). If one unit detects a promising sound, then others can be moved nearby to "triangulate" the source.
  • The ARU was invented at the Cornell Lab of Ornithology's Bioacoustics Research Program, and has been used to record everything from right whales in the North Atlantic to Africa's forest elephants. The 2002 search for the ivory-bill in Louisiana's Pearl River Wildlife Management Area used 12 ARUs. The 2004-05 Arkansas search, across a larger area, required 24 ARUs.

In deciding where to deploy the recording units, the Cornell surveyors say they try to think like a hungry ivory-bill or an egg-laying beetle, asking themselves: Where are the dying trees with beetle larvae under the bark, where a woodpecker might feed? They use infrared aerial photography to find what they call "pink (in infrared rendition) bathtub rings" of stressed trees around pools of water. If beetle larvae are under the bark of the stressed trees, then maybe--just maybe--Ivory-billed Woodpeckers will find the trees, tear off the bark in a process called scaling, and feed.

The surveyors in the field won't know if their think-like-a-beetle strategy is paying off until the recordings are analyzed in the Ithaca laboratory, and that is a daunting task:

  • Recordings from the 2004-05 survey (eight hours a day from each of 24 units) are the equivalent of nearly three years of continuous recording. Even a major university like Cornell can't spare enough people to listen to all that sound, so the first part of the detection process is automated.
  • Data from the search, when it is finally analyzed and archived, will add up to a server-busting 3 or 4 terabytes (3 or 4 thousand billion bytes). All recordings are saved, even if nothing is found in the first analysis.
  • Two Cornell-developed sound visualization and measurement programs, called XBAT and Raven, are used. XBAT (for eXtensible BioAcoustic Tool) rapidly scans the digital recordings and detects sounds similar to those made by ivory-bills. Raven is used for interactive exploration of sounds that are of particular interest.
  • On the sound spectrograms, XBAT highlights sounds of interest with colored boxes to catch the eye of the analyst. The analyst plays and replays each detected event, sometimes comparing the sound to reference recordings of ivory-bills, their close relatives, or other species that sound almost alike. As the search neared the midpoint, analysts had already scrutinized 91,000 "detections," and set aside all but a handful.

Archival recordings of Ivory-billed Woodpeckers are critical to the detection software and the human analysts. Although there are no confirmed records of the ivory-bill's double-rap, the bioacoustic scientists have good-quality recordings of vocalizations from the 1935 expedition to Louisiana's Tensas River by Cornell ornithologists Peter Paul Kellogg, Arthur A. Allen, George Sutton and James T. Tanner. On a film sound-track the 1935 team successfully recorded an adult pair of ivory-bills and their young at the nest. This recording--the first ever made of the voice of the ivory-bill--provides the "template" that the detection software attempts to match, and guides the decisions of the analysts about which sounds are the most interesting.

"Automation is great," Charif says, "but the real gold standard, in the final analysis, is still the human ear."

Nevertheless, this veteran of earlier bioacoustic searches--before XBAT and other tools became available--wants to save human ears for what they do best. He observes that if ivory-bill survey relied on human ears alone, those humans would have to remain attentive through tens of thousands of hours of recordings--to catch the few seconds that the ivory-bill searchers are looking for. By then, the results would be too late to be of any real use.

"With XBAT," Charif says, "we enable our trained, expert ears to concentrate on the relatively tiny percentage of the entire data set that really warrants their attention."

Read a BirdScope article by Russ Charif about the acoustic search