David Sibley, a renowned birder, visited Cornell's Lab of Ornithology last year to view the blurry April 2004 video of an ivory-billed woodpecker, thought to be extinct for 60 years. The four-second video is the cornerstone evidence for Cornell's claim that the ivory-bill is still alive in the southeastern United States.

Sibley, author and illustrator of the best-selling "The Sibley Guide to Birds," has now challenged the Lab's analysis of the footage, known as the Luneau video, saying the image could be a black-and-white pileated woodpecker, not the ivory-bill.

The debate that began with the release of the video in April last year has vaulted to the highest scientific stage, with a peer-reviewed challenge headed by Sibley in the March 17 issue of Science (Vol. 34) and a response by Lab of Ornithology Director John Fitzpatrick.

"It's almost like a gentlemen's duel at this point," said Ken Rosenberg, director of the conservation science program at the Lab, a co-author of the paper refuting Sibley's analysis. "It is fascinating that we could look at this together and have such huge differences in interpretations in what we are seeing."

Cornell experts maintain that the pattern of white in the wings and how the bird flies in the video leave no doubt that the bird is an ivory-billed woodpecker.

In one segment of the video, the black-and-white edge of a bird can be seen peeking out from behind a tree. While Cornell experts believe the image shows an ivory-bill perched on the back side of the tree with the black top and white bottom of a closed wing sticking out, Sibley and his colleagues suggest it shows an open-winged pileated woodpecker preparing for flight.

In such a pose, the underside of a pileated woodpecker's wings would appear black-rimmed with white interior. The Cornell experts assert that pileated woodpeckers have far more black all along the wing's edges and far less white than the video shows. Also, they argue the image does not show any black on much of the lower portion of the wing.

Further, Sibley and his team believe that the white seen on the wings of the bird flying away from the camera shows the patch of white on the underside of a pileated woodpecker, and that the wings were twisting in flight, hiding a pileated's mostly black upper side. The Cornell experts counter by claiming the video sequence shows all the classic ivory-bill markings -- extensive white on the top side of the bird's wings, black wing tips and no black along the rear edge of the wing. Also, they add, the idea that the wings are twisted in flight contradicts all models and photographic analyses of how birds flap their wings.

"There isn't anything out there that makes us think you can see the underside at all times in the flight," said Rosenberg. "Every single frame is a white-winged bird with a black wing tip."

The Lab of Ornithology team also commented that the filmed bird flies in a direct, straight manner similar to a pintail duck and characteristic of an ivory-bill, while pileated woodpeckers flap with an undulating motion.

"The most important thing is the flight of the bird," said Rosenberg of the video. "That thing barrels off the tree. A pileated does not fly that way."

While Sibley and colleagues claim the video evidence is not definitive proof, they do not deny that ivory-billed woodpeckers may well exist in the Big Woods of Arkansas, where the video was recorded. The experts from both sides of the debate are also committed to the conservation efforts since the woodpecker's announced rediscovery in April 2005.

Currently, over 20 scientists and rotating groups of highly skilled volunteers are searching for woodpeckers within the half-million acres of Arkansas' Big Woods. The searchers are hopeful they will find more evidence of the ivory-bill now that the more active breeding season is about to begin.

Cornell's Lab of Ornithology has posted links to the Science articles and video analysis of the Luneau video on its Web pages for the ivory-billed woodpecker project at http://www.birds.cornell.edu/ivory.