The Cornell researchers announced the results at the annual meeting of the American Ornithologists' Union in Santa Barbara, Calif., Aug. 24, and they have invited the public to listen to the calls and knocks on the Web at http://www.
The recordings reveal sounds that, experts say, are strikingly similar to those made by ivory-billed woodpeckers and provide compelling information that can be added to evidence already gathered of the bird's existence. One of the recordings, from Jan. 24, 2005, captured a distant double knock, "Bam bam!" followed by a similar and much closer double knock 3.5 seconds later -- possibly the drumming displays of two ivory-billed woodpeckers communicating with one another by rapping on trees.
"I immediately felt a thrill of excitement the first time I heard that recording," said Russell Charif, a bioacoustics researcher at the Cornell Lab of Ornithology. "It is the best tangible evidence so far that there could be more than one ivory-bill in the area."
Only a single bird at a time was documented through sightings and video footage during a yearlong search of the area in 2004-05. Whether more than one bird exists is still a big question, Charif said, but quantitative analyses of the most promising sounds indicate a high probability that they were made by ivory-billed woodpeckers.
The sounds were recorded on autonomous recording units (ARUs) designed by the Cornell lab and strapped to trees in the swamp. More than 150 sites were monitored in the half-million-acre "Big Woods" of Arkansas.
After eliminating thousands of noises from gunshots and other sources, the researchers found about 100 double knocks that bear a strong resemblance to the display drumming of the ivory-bill's closest relatives. The sounds were clustered around certain recording locations at certain times of day -- a pattern that would not be expected if they had been produced by random noises.
Then ARUs also recorded nasal tooting calls similar to those of ivory-billed woodpeckers. Charif said blue jays are notorious vocal mimics that sometimes utter calls like those of ivory-bills. However, he added, a sophisticated acoustic classification program categorized nearly all of the unknown calls from Arkansas as most similar to ivory-billed woodpecker recordings. None matched up with "tooting" calls of blue jays from the Cornell lab's audio collection, but the researchers say they cannot rule out blue jays until they analyze more variants of the calls.
"We're excited and encouraged by the acoustic analysis," said John Fitzpatrick, director of the Cornell Lab of Ornithology. "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."
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 Nov. 1.