The sharp, powerful rap-rap burst out of the dense oak woodland around them again, carrying across still swamp waters as the team of biologists scrambled to capture it on state-of-the-art recording equipment.

Less than a mile away, another team of experts scouring the 35,000-acre (14,000-hectare) Pearl River wildlife refuge in southeastern Louisiana heard the same double-rap signal and saw two birds flash through the treetops in a chase.

Had they found their quarry and confirmed what could be the greatest ornithological find of modern times?

Ever since a turkey hunter claimed to have seen a pair of Ivory-billed Woodpeckers in the spring of 1999, just 40 miles (65 km) from the bustling city of New Orleans, the wildlife community has been trying to confirm the existence of the species labeled "probably extinct" and last seen in 1941.

The six scientists and wildlife experts prowling the Louisiana bayou for four weeks in January and February didn't come up with the definitive proof, namely, a photograph.

But there is a growing consensus that the species known in Latin as Campephilus principalis, which has gone unrecorded for 60 years and was recently removed from birding field guides, has survived in spite of the intense logging blamed for its extinction.

Tantalizing clues suggest that America's largest and most visibly vibrant woodpecker - with its bright red cockade, huge ivory-colored bill and 3-foot (1-metre) wingspan - may not have vanished. But the tangled forest of the Pearl River Wildlife Area on the Mississippi border stubbornly refused to loosen its protective embrace and reveal a positive glimpse to the searchers.

The loud, gunshot-like tree-rapping, uncharacteristic of other woodpeckers and never before heard by any of the experts wading through the Louisiana bayou, together with evidence of distinctive bark-peeling associated with the feeding habits of the Ivory-billed Woodpecker and fresh, large nesting cavities have convinced some skeptics the bird is clinging to life.

PRECISION SEARCH

David Luneau, an associate professor of engineering technology at the University of Arkansas at Little Rock, who was one of six people hand-picked by the state wildlife service to mount the precision-planned search, thinks it has survived its prematurely declared death.

"I believe that the Ivory-billed Woodpecker is not extinct," Luneau said after his fourth and most exhaustive search, sponsored by binocular and sports optics-maker Zeiss.

"I am more convinced now than ever before that the woodpecker has survived for 50 years under the radar screen of the world of birding and ornithology," he added, noting that the rapping sound, a means of communication, was consistent with key research data about the bird from 1941.

But without irrefutable proof, such as a photograph, the guardians of America's ornithological records will not declare the Ivory-bill a reclaimed member of the U.S. birding family.

Luneau, however, thinks that time may not be too far off.

"I really have a sense that it's not going to be long before people come up with the evidence," he said in a telephone interview, explaining that more searches were planned and hunters were being asked to take cameras into the forest.

His belief that the bird survived the intense logging in its primary habitat, Louisiana's hardwood forests, is based on the sounds, bark-scaling and persistent claim of the young turkey hunter who ignited one of the biggest dramas in American birding.