What's that knocking? Sound evidence offered for long-lost woodpecker.

Cornell University's Laboratory of Ornithology has released recordings from Arkansas of possible calls and drumming by one or more ivory-billed woodpeckers.

The release will fuel a new round in the debate over whether the bird, long considered extinct, still makes its home in the state's Big Woods area. Last April, a consortium including the Cornell lab announced sightings of at least one bird (SN: 5/7/05,p. 291), but some other scientists challenged the evidence.

The sounds, recorded by digital devices left in the woods for weeks at a time, reveal what the Cornell scientists say could be the sharp calls, sounding like 'kent,' of an ivory-billed woodpecker and the species' distinctive double knocks on trees. The researchers gleaned the recordings from some 18,000 hours of data collected at more than 150 spots in the Arkansas woodlands.

The recordings are posted on the lab's Web site (www.birds.cornell.edu) along with recordings of the woodpecker from the 1930s.

For the past 60 years, bird-watchers have occasionally reported seeing this Elvis-in-feathers, though the claims have inspired more controversy than confidence. The strongest recent evidence: seven sightings and 4 seconds of blurry video from the Cache River National Wildlife Refuge in Arkansas.

As the euphoria faded, three scientists not on the search team challenged the video as insufficient proof. "It simply did not look like an ivory-billed," says Jerome Jackson of Florida Gulf Coast University in Fort Myers, who has published a book about the search for the species. Jackson, Richard Prum of Yale University, and Mark Robbins of the University of Kansas in Lawrence proposed that the bird in the video was probably a pileated woodpecker.

The Cornell researchers gave the three biologists a preview of the sound recordings, and the skeptics withdrew a critical manuscript that they had submitted for publication. The recordings, at first listen, "sound like an ivory-billed woodpecker," says Jackson. However, he says, "we stand by our questions regarding the video."

Cornell's Ron Rohrbaugh says that the challenge prompted lab analysts to take another look at the video, which has "bolstered" confidence that it shows an ivory-billed woodpecker.

The Cornell team presented the recordings this week in Santa Barbara, Calif., at the annual meeting of the American Ornithologists' Union.

Cornell's Russell Charif calls two of the recordings, from January 2005, the "first tangible hint so far" of more than one individual ivory-billed woodpecker. The recording includes what sounds like the species' distinctive double knock in the distance, and, some 4 seconds later, a much closer knock knock.

Acoustic analysis indicates that the several calls posted on the Web site resemble 1930s recordings of ivory-billed woodpeckers. However, the Cornell researchers say that they haven't so far ruled out atypical blue jay "tootings."

Jackson predicts that only clear photographs or videos will settle the debate over the ivory-billed woodpecker. "The conclusive proof is yet to come," he says.