Three biologists are questioning the evidence used by a team of bird experts who made the electrifying claim in April that they had sighted an ivory-billed woodpecker, a bird presumed to have vanished from the United States more than 60 years ago, in the swampy forests of southeast Arkansas.
If the challenge holds up, it would undermine not only a scientific triumph - the rediscovery of a resplendent bird that had been exhaustively sought for years - but also significant new conservation expenditures in the region.
The paper questioning the discovery has been submitted to a peer-reviewed journal, which could post the analysis online within a few weeks. But the paper will be accompanied by a fierce rebuttal by the team that announced the discovery, and a response to that rebuttal by the challengers.
The expected publication of the paper and the rebuttal was confirmed in interviews and e-mail exchanges with two authors of the challenge, Richard O. Prum and Mark B. Robbins, ornithologists at Yale and the University of Kansas, as well as with two members of the team that reported finding the woodpecker.
The third author of the new paper is Jerome A. Jackson, a zoologist at Florida Gulf Coast University and the author of the book, "In Search of the Ivory-Billed Woodpecker," published in 2004.
"In my opinion," Mr. Jackson wrote in an e-mail message on Wednesday, "the data presented thus far do no more than suggest the possibility of the presence of an ivory-billed woodpecker. I am most certainly not saying that ivory-billed woodpeckers are not out there. I truly hope that the birds do exist in Arkansas or elsewhere and have been championing this idea for a long time."
Both groups of scientists declined to name the journal or to discuss the details of the challenge and the response until they were published.
But they made it clear that the debate revolves around four seconds of fuzzy videotape that, by chance, captured a bird with sweeping white-and-black wings as it darted from its perch on the far side of a tupelo tree in April 2004 and flicked over swampy waters before vanishing in the trees 11 wing beats later.
That video clip was just one piece in a pile of drawings, recordings and other evidence collected in more than a year of searching and deploying cameras and listening devices across the vast swampy reaches of the Cache River National Wildlife Refuge.
Altogether, the original research team, led by scientists from Cornell University and the Nature Conservancy, compiled seven sightings, including the video, as well as recordings of a "double knock" sound typical of the ivory-billed bird.
But only the video was potentially solid enough to confirm for the wider ornithological community the existence of the bird, the authors said in various statements at the time.
Everyone agrees that the bird that appears on the tape is either an ivory-billed woodpecker or a pileated woodpecker, a slightly smaller bird that is relatively common. Both species have a mix of white and black plumage. However, the ivory-billed woodpecker has a white trailing edge to its wings while the pileated woodpecker has a black trailing edge.
The team that conducted the original search for the bird ran extensive tests, including recreating the scene captured in video using flapping, hand-held models of the two types of woodpecker. They concluded that the plumage patterns seen in the grainy image could only be that of the ivory-billed woodpecker.
The authors of the new paper disagree.
Only extended scientific discussion - or new pictures of the bird from additional searches - will determine whose view will prevail. Another intensive scientific search of the region is scheduled to begin in November, Cornell officials said.
"The people who originally announced this thoroughly believe they got an ivory-billed woodpecker," Dr. Robbins said in an interview. Determining if a species has crossed the threshold of extinction often requires decades of observation to ensure that no stray individuals have found a reclusive hideaway.
Supposedly extinct species have been rediscovered with some frequency over the last century. One famed example is the coelacanth, a fish known only from fossils for generations but then caught by African anglers.
In the case of the ivory-billed woodpecker, a magnificent bird with a 30-inch wingspan and a red crest, determining that it has not become extinct has proved equally daunting. Individual birds were widely dispersed, and the woodpecker shared habits and habitat with the pileated woodpecker.
Van Remsen of Louisiana State University, an expert on the woodpecker and a member of the team that reported finding the ivory-billed species, said he remained confident of the discovery.
"We can counter everything," he said. "We stick to our guns."
The announcement of the bird's apparent discovery came on April 28, when the scientists' findings were published in the online version of the journal Science.
The announcement thrilled conservationists, who saw the bird as the perfect symbol around which to build an invigorated protection plan for woodland habitat in the Southeast, which harbors a rich array of wildlife and plants.
The Bush administration used the reported sightings in Arkansas to promote its "cooperative conservation" philosophy. The day the rediscovery was publicized, the administration announced a variety of initiatives, including a plan to pay more than $13 million to landowners within the region's floodplains who plant and maintain forests.
John W. Fitzpatrick, the co-leader of the search for the bird and director of the Cornell University Laboratory of Ornithology, said it was normal for scientists to disagree about evidence of this sort, especially because in this case the video in question was "pretty crummy."
But he said that extensive analysis was done and redone to eliminate the possibility that the bird was a pileated woodpecker.
Dr. Fitzpatrick added that there was "significant additional evidence right now" that would be published in coming months.
He declined to comment on the challengers' assertions, saying any discussion could jeopardize publication of the exchange of papers on the video.