|Technician Chris Tessaglia-Hymes
identified the recorded sounds by listening to them and inspecting
the spectrographic images.
Diane L. Tessaglia-Hymes
In February 2002, a six-member search team funded by Zeiss Sports
Optics emerged from the Louisiana bayous, unrewarded but hopeful.
They had not found an Ivory-billed Woodpecker during a month of
searching, but they had heard and tape-recorded several loud double
raps that were reminiscent of the species long feared to be extinct.
Meanwhile, we hoped that 12 autonomous recording units (ARUs) deployed
by the Lab's team would capture the sounds of the magnificent woodpecker
if it still existed there.
We recovered 4,146 hours of data recorded at 12 dispersed locations
in the bottomland forests of the lower Pearl River, Louisiana. Sadly,
after scrutinizing more than 130,000 sounds deemed most likely to
have been produced by an ivory-bill, we can state with some confidence
that no Ivory-billed Woodpeckers were present in the area between
January 25 and March 17, 2002.
Although we are disappointed to have found no evidence of ivory-bills
in what we believed to be the most promising terrain, we are pleased
that the ARUs functioned as intended. Each one recorded sounds unobtrusively
and continuously during two four-hour sessions each day. Our analysis
also allowed us to pinpoint and examine the mysterious double rap
recorded by the Zeiss team.
|Each sound produced
by a bird leaves a recognizable signature in a sound spectrogram.
Above, (a) Ivory-billed Woodpecker vocalizations and knocks
from the 1935 recording, (b) Pileated Woodpecker wek wek wek
contact notes, (c) Red-shouldered Hawk keeyuur calls, (d)
Carolina Wren song.
With detection software developed here at the Lab, our technician,
Chris Tessaglia-Hymes, could analyze the recordings in a matter
of months, compared with the two years that would have been required
otherwise. Our program detected and marked all sounds that could
have been made by an ivory-bill, based on the only Ivory-billed
Woodpecker recordings in existence, made by Lab founder Arthur Allen
and codirector Peter Paul Kellogg.
For each of the 12 ARUs, automatic detection analysis identified
thousands of sounds that had some Ivory-billed Woodpecker characteristics.
This level of "false positive" was desirable because we
preferred to search through many such erroneous detections rather
than set the parameters too tight, risking that a true positive
might not be identified.
Chris inspected the spectrographic image of each sound detection
on a computer screen and listened to audio playback through headphones.
The most common detections included songs and calls of Carolina
Wren (Thryothorus ludovicianus
), Tufted Titmouse (Parus
), Red-shouldered Hawk (Buteo lineatus
Woodpecker (Dryocopus pileatus
), Red-bellied Woodpecker (Melanerpes
), spring peeper (Hyla crucifer
), and gunshots.
We paid special attention to the acoustic events of Sunday afternoon,
January 27, a day when the Zeiss team and our team independently
heard a double-rap sound reminiscent of the display drumming of
the Ivory-billed Woodpecker. We carefully inspected and listened
to the acoustic signals detected by ARU #10, the closest unit, and
easily identified the time period coinciding with the sounds we
The signals in question had an explosive onset equally represented
by a wide band of frequencies, followed by a tapering "smudge."
This signature is clearly that of a gunshot, with the smudge representing
decaying reverberations of the sound through the forest. We detected
the same sound, although faint, at ARU #9, a distance we deemed
too far for a woodpecker drum-rap, of any species, to travel through
a full-canopy bottomland forest and still be detected by our ARUs.
These acoustic signatures exactly matched those of hundreds of other
presumed gunshots recorded by our ARUs and dashed all hopes that
the mystery sounds would prove to be the long-lost woodpecker.
Although we concluded that no Ivory-billed Woodpeckers were present
during the period monitored, we still view it as possible (though
the chances are slim) that one to several pairs of ivory-bills could
be using portions of the Pearl River forests outside the scope of
our search. Ivory-billed Woodpeckers were highly mobile, sometimes
moving great distances as their principal resources (large, recently
dead trees) became available or aged beyond use. The bottomland
hardwood forests of the lower Pearl River are extensive, and they
are in better condition to support large woodpecker populations
today than they have been for a century or more.
There have been no confirmed sightings of Ivory-billed Woodpeckers
in the United States for more than 50 years, but there have been
persistent accounts bearing a reasonable level of credibility. The
history and profile of this great bird, the tragedy of its loss,
and the importance of any potential rediscovery all demand a truly
concerted effort by scientists willing to conduct a careful and
objective search. We hope to contribute to that effort by conducting
at least one more search using ARU technology at a new location
in the future.