What Do the Experts Think?

August 24, 2005
So, were the calls and double knocks in the Big Woods of Arkansas made by Ivory-billed Woodpeckers? After analyzing more than 17,000 hours of recordings, experts at the Cornell Lab of Ornithology agree that the sounds are strikingly similar to those of Ivory-billed Woodpeckers. However, they still cannot say with certainty whether ivory-bills made the sounds.

Preliminary Wrap on the Double Raps

Because there are no recordings of known Ivory-billed Woodpecker double knocks with which to compare the sounds from Arkansas, the research team listened to recordings of related woodpeckers from Central and South America for clues about what an ivory-bill’s display drumming might sound like. After eliminating thousands of noises produced by breaking tree branches, raindrops, mechanical noises, gunshots, foraging taps and drumming displays made by other woodpeckers, the researchers found about 100 tantalizing double knocks that bear a strong resemblance to those of the ivory-bill’s closest relatives. Equally tantalizing is the fact that these sounds were clustered around certain recording locations at certain times of the day—a pattern that would not be expected if the sounds were produced by random noises.

The recording of a distant double knock followed by a single knock on January 24, 2005, is especially intriguing. Russell Charif, who leads the acoustic research effort, first heard the recording with woodpecker expert Martjan Lammertink. “I immediately felt a thrill of excitement when I heard the recording,” Charif said. “Martjan looked at me and said something understated, like, ‘That sounds really good!’” Charif said, however, that as he listened to the sound repeatedly, his excitement was tempered as he began to wonder about slight differences in the sound compared with scientific descriptions of Ivory-billed Woodpecker display drums.

For example, those descriptions mention that the first knock is louder than the second. In the recording from Arkansas, the second knock is slightly louder. However, there are so few records of these display drums that researchers do not know to what degree they may vary depending on the geographic location and the context in which they are used. Without any recordings of known double knocks, there is no precise reference with which to compare the mysterious sounds from the Big Woods.

Ivory-bills or Imposters?

Some of the double knocks were recorded in the same area as nasal calls that closely resemble the only known recordings of Ivory-billed Woodpeckers, made 70 years ago. However, observers in Arkansas have heard jays uttering long sequences of nasal tooting calls that sound like Ivory-billed Woodpeckers. Charif said that a sophisticated acoustic classification program categorized nearly all of the unknown calls from Arkansas as most similar to Ivory-billed Woodpecker recordings. A few of the notes also resembled those of White-breasted Nuthatches, a species that utters a weaker version of the tooting call. None of the sounds matched up with “tooting” calls of Blue Jays from the Lab’s extensive collection of recordings. However, the research team isn’t ruling out Blue Jays yet. They would like to analyze more calls to get a better sample of the variants of the “kent”-like calls made by jays. To aid in the effort, birders can contribute recordings to The Blue Jay Challenge.

Sounds Good

Researchers at the Lab say that the sounds from the Big Woods of Arkansas are intriguing and that the quantitative analyses indicate a high probability that they were made by Ivory-billed Woodpeckers. “When we listen to the recordings from the Big Woods and look at the sonograms, our ears hear and our eyes see evidence of sounds like those of Ivory-billed Woodpeckers,” said Ken Rosenberg, conservation science director at the Cornell Lab of Ornithology. “It’s nice to have quantitative analyses confirm that our impressions are not just wishful thinking.”

During the year-long search in 2004-2005 only a single bird at a time was documented in seven sightings and brief video footage. The recording of close and distant double knocks are the best tangible evidence so far that there could be more than one Ivory-billed Woodpecker in the area. Rosenberg emphasizes that the recordings do not conclusively confirm the presence of additional Ivory-billed Woodpeckers, but that the results warrant continued research and conservation efforts.

Meanwhile, a team led by the Cornell Lab of Ornithology, The Nature Conservancy, the U.S. Fish and Wildlife Service, the Arkansas Game and Fish Commission, and other partners, will renew search efforts in the Big Woods beginning on November 1, 2005.

 “We’re excited and encouraged by the acoustic analysis,” said John Fitzpatrick, the Lab’s director. “These sounds give us additional hope that a few Ivory-billed Woodpeckers do live in the White River and Cache River region. But this species is still on the verge of extinction and huge mysteries remain to be solved. Certainly, we have a lot more work to do before we know enough to determine its population size, let alone ensure its survival.”