By PEGGY HARRIS For the last three years, researchers in camouflage and waders have slogged through the east Arkansas woods hoping to spot a rare bird that so far seems unwilling to be seen. Some scientists still believe the ivory-billed woodpecker exists in the Big Woods, but they haven't been able to capture a sharp image of its remarkable 30-inch wing span and glossy black and white feathers on film or video camera. To date, searchers have investigated about 83,000 of the 550,000-acre woods that swallow up the Cache River National Wildlife Refuge. The refuge is where kayaker Gene Sparling spotted the bird February 11, 2004, and Cornell University experts said they made subsequent sightings. Engineer David Luneau caught a blurry image of what some believe is the "Lord God bird," the third-largest woodpecker in the world, on video in 2004. Others challenge the claim that the ivory bill survived decades of clearing forests for farming, timber, roads and towns. "Since early 2005, none of our group nor anyone from the public that we are aware of has made a definitive absolute sighting of an ivory-billed woodpecker that we can document with a photograph or a sound recording," said Ron Rohrbaugh, project director at the Cornell Lab of Ornithology. According to researchers, it became known as the "Lord God bird" because people, upon seeing it, would exclaim, "Lord God, look at that bird!" The Big Woods is a hot, humid place