New recordings have been released which scientists believe bolster the case for the existence of the ivory-billed woodpecker, once thought extinct.
The last confirmed sightings were in the '30s and '40s (David Allen)
News that the North American bird had been sighted in Arkansas for the first time since the 1940s caused a sensation when it was announced in April.
But some ornithologists doubted the claims and said more proof was needed.
Now Cornell University researchers have released sounds of what they believe to be the bird's distinctive double knock.
They hope these and other recordings made in the swampy "Big Woods" of eastern Arkansas will help to convince the sceptics of the woodpecker's continued existence.
The Cornell scientists announced the results of their sound study at the annual meeting of the American Ornithologists' Union in Santa Barbara, California.
The bird noises were picked up on autonomous recording units strapped to trees at more than 150 sites in the half-million-acre Big Woods.
The team sorted through over 18,000 hours of material, eliminating noises from gunshots and other sources, to find about 100 double knocks that bear a strong resemblance to the display drumming of the ivory-bill's closest relatives.
The sounds were clustered around certain recording locations at certain times of day - a pattern that would not be expected if they had been produced by random noises, the scientists say.
One of the recordings, from 24 January this year, captured a distant double knock, followed by a similar and much closer double knock 3.5 seconds later - possibly the drumming displays of two ivory-billed woodpeckers communicating with one another by rapping on trees.
"I immediately felt a thrill of excitement the first time I heard that recording," said Russell Charif, a bioacoustics researcher at the Cornell Lab of Ornithology. "It is the best tangible evidence so far that there could be more than one ivory-bill in the area."
Then ARUs also recorded nasal tooting calls similar to those of ivory-billed woodpeckers.
"We're excited and encouraged by the acoustic analysis," said John Fitzpatrick, director of the Cornell lab. "These sounds give us additional hope that a few ivory-billed woodpeckers do live in the White River and Cache River region."
The "stunning" red, white and black woodpecker was once distributed across much of the south-eastern US and Cuba, until logging and forest clearance for agriculture restricted its range.
By the 1920s, it was assumed to be extinct, although, in 1944, there was one more confirmed sighting in North America of a lonely unpaired female.
The ARUs were strapped to trees
From then, decades of searches yielded nothing - until a brief, blurry video shot recorded last year picked up what many experts believe to be a genuine sighting of the "lost" woodpecker.
Some ornithologists have called for more definitive evidence - and the Cornell lab believes the audio files go a long way to meeting the scepticism.
"The thrilling new sound recordings provide clear and convincing evidence that the ivory-billed woodpecker is not extinct," said Richard Prum of Yale University in a statement earlier this month. Prum had previously stated doubts about the bird's existence.
The Cornell scientists have invited the public to listen to the calls and knocks on their web page.