Martjan Lammertink The Ivory-billed Woodpecker has been rediscovered in the Big Woods of Arkansas Zoom In

Knock knock, who's there?

26-08-2005

The recent rediscovery of one of the world’s most enigmatic birds appears to stand up to scientific scrutiny after additional, vital acoustic evidence was presented by the ornithologists who made the sightings.

In April 2005, a team of scientists led by John Fitzpatrick from the Cornell Laboratory of Ornithology announced in the journal Science that the Ivory-billed Woodpecker Campephilus principalis had been sensationally rediscovered in the Cache River National Wildlife Refuge, Arkansas, USA. A crucial piece of evidence was a blurry video-tape of a woodpecker flying away from the camera.

Some ornithologists had questioned their interpretation of this video, saying it was inconclusive. However, the subsequent release of tape recordings made in Arkansas of the characteristic nasal “kent” calls of the species plus two double-knocks on a tree, interpreted as being made by Ivory-billed Woodpeckers has satisfied most sceptics of the species' continuing existence.

Over 18,000 hours of sounds were recorded on autonomous recording units (ARUs) designed by the Cornell Lab and strapped to trees in the swamp. More than 150 sites were monitored in the half-million acre Big Woods of Arkansas and these have now been analysed by researchers. Most striking is a recording from 24 January 2005 which captures a distant double knock, followed by a similar and much closer double rap 3.5 seconds later – possibly the drumming displays of two Ivory-bills communicating with one another by rapping on trees.

"I immediately felt a thrill of excitement the first time I heard that recording. It is the best tangible evidence so far that there could be more than one ivory-bill in the area." —Russell Charif, Bioacoustics Researcher, Cornell Lab of Ornithology

After eliminating thousands of noises from gunshots and other sources, the researchers found 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.

Then ARUs also recorded nasal tooting calls similar to those of Ivory-billed Woodpeckers. However, Blue Jays Cyanocitta cristata are notorious vocal mimics that sometimes utter calls like those of ivory-bills. However, a sophisticated acoustic classification program categorised nearly all of the unknown calls from Arkansas as most similar to Ivory-billed Woodpecker recordings. None matched up with "tooting" calls of Blue Jays from the Cornell's audio collection, but researchers say they cannot rule out the jays until they analyze more variants of the calls.

"These sounds give us additional hope that a few Ivory-billed Woodpeckers do live in the White River and Cache River region. But this species is still on the verge of extinction and huge mysteries remain to be solved. Certainly, we have a lot more work to do before we know enough to determine its population size, let alone ensure its survival." —John Fitzpatrick, Director, Cornell Lab of Ornithology

A team led by the Cornell Lab of Ornithology, The Nature Conservancy, the U.S. Fish and Wildlife Service, the Arkansas Game and Fish Commission and other partners including Audubon (BirdLife in the US) will renew search efforts in the Big Woods in November. Hopefully the lack of leaves on the trees should hopefully prove more fruitful for searching at that time of year.

The calls are available to listen to on the Cornell Lab's web site.