Ivory-billed Woodpecker is extinct, say two teams of researchers

This article was first published on our blog on February 2, 2012. —MM

Two groups of scientists studying museum specimens and sightings of the Ivory-billed Woodpecker have reached the same disappointing conclusion: The iconic bird is extinct. Each team described their findings in papers in the February 2012 issue of the journal Conservation Biology.

1 in 15,625

The first team, led by biologist Nicholas Gotelli of the University of Vermont, studied 239 Ivory-bill specimens collected from 1853 to 1932. The researchers concluded that the chance that the species persists today is 0.0064 percent. Said another way, the odds of finding a living Ivory-bill are less than 1 in 15,625.

Gotelli and his colleagues also considered whether additional searches would improve the probability of discovering the Ivory-bill. They evaluated evidence from searches conducted by the Cornell Lab of Ornithology in 2006-07 “at four sites deemed to be among the most promising for relictual populations” of the bird: the Congaree River in South Carolina; the Choctawhatchee River in Florida; the Pearl River in Louisiana and Mississippi; and the Pascagoula River in Mississippi.

The Cornell team searched for Ivory-bills and censused all the bird species at each site, counting a total of more than 31,000 individuals of more than 55 species. The numbers of species and individuals the Cornell group encountered can help answer the question of whether further searches are warranted, Gotelli writes. His paper defines “a practical stopping rule for deciding when to abandon the search in a particular site.” It’s a question of assessing the costs of conducting searches with the potential reward of finding one more species.

“A simple, empirical stopping rule is to stop searching when each observed species is represented by at least two individuals,” Gotelli writes. The formula means that further searches at the Congaree River site are not warranted because the Cornell team reported observing more than 15,000 birds, and each of the 56 species it reported were found at least twice. The birds seen and heard at the other three locations included a handful of species encountered only once, but, Gotelli writes, “the probability that the next individual censused would represent a new species was very low.”

Gotelli’s specimen analysis also considered when the Ivory-bill would have gone extinct based on hypothetical population sizes from 1929 to 1932. A population of 100 birds would have lasted no longer than 1980, and a population of 1,000 birds would have gone extinct by 1996. To have survived until 2008, Gotelli suggests the population would have had to be 50,000 in 1932. Ivory-bill expert James Tanner, in his seminal report The Ivory-Billed Woodpecker, estimated that the late 1930s population was in fact 22 birds.

29 so-called uncertain sightings

A second team of investigators, led by Andrew Solow, a senior scientist at the Woods Hole Oceanographic Institution, analyzed 29 so-called uncertain sightings of the species since 1946, including headline-generating reports from the Pearl River in 1999, Cache River NWR in Arkansas in 2004 and 2005, and the Choctawhatchee River in 2005. Solow’s paper describes a statistical method that “is the first to treat uncertain sightings in a formal way, neither simply excluding them nor simply treating them as valid.” Solow’s group concludes that “the Ivory-billed Woodpecker is extinct,” but the researchers don’t provide an estimate for when extinction occurred.

The new papers follow three studies published in 2010 that also found the species is almost certainly extinct. They estimated extinction to have occurred by 1948, 1965, and, if recent controversial sightings were accepted, 2010.

Gotelli says his team and Solow’s worked independently but reached a similar conclusion.

“It is easy enough for skeptics to pick apart a single statistical analysis,” he says. “Statistical analysis of non-experimental data is, after all, an indirect form of inference, and the results always depend on the assumptions of the model and the kind of data that are used. But in this case, you have two independent teams that are using different data sets (for the historical analysis, we excluded everything except for valid, dated, geo-referenced specimens, whereas Solow’s analysis included other kinds of observations, such as photographs) and completely different statistical models, but they arrive at the same qualitative conclusion (persistence is improbable). These analyses are also consistent with earlier statistical studies published by Chris Elphick, Dave Roberts, and other colleagues. Taken collectively, the results of these multiple independent investigations make a powerful case that the Ivory-billed Woodpecker is extinct.”

I asked two well-known Ivory-bill searchers, Jerome A. Jackson and Bobby Harrison, for their reactions to Gotelli and Solow’s papers.

Jackson, a professor and the former Whitaker Eminent Scholar in Science at Florida Gulf Coast University, is the author of In Search of the Ivory-Billed Woodpecker and a June 2002 Birder’s World article, “The Truth Is Out There.” In 2006, he famously referred to Cornell’s claims about sightings in Arkansas as “faith-based ornithology.”

“I think their conclusions are fairly sound and their approaches interesting and complementary,” Jackson says of Gotelli and Solow. “Both have to be taken at face value and with the understanding that assumptions of other factors remaining constant over time simply are not met. Lots of things change over time: the number of competent scientists/birders in the field, the quality of optics, accessibility of habitat, etc. That’s the weakness in such modeling.

“Other factors, if added in to these studies, might suggest an even higher probability of extinction,” he adds. “For both studies, I think there is a social/psychological factor involved — all of those sightings in the 1960s and again in 2004-2005 were not independent, and following an initial report each time, later sightings were certainly influenced by earlier ones. People see what they want to see or what they believe to be out there.”

‘Doesn’t negate what I’ve seen’

Harrison is a nature photographer and professor of photography at Oakwood University in Alabama. Along with kayaker Gene Sparling and Living Bird Editor Tim Gallagher, Harrison claimed to have seen an Ivory-bill in Arkansas in 2004. Their sighting spurred the Cornell Lab to launch a large search and to report in the journal Science that the bird was still alive.

“They can do all the math they want, but that doesn’t negate what I’ve seen,” Harrison says of the recent papers. “I have seen Ivory-bills six times and have seen at least two different birds.”

Still, he doesn’t blame anyone for not believing his reports. “If I hadn’t seen them myself, I may have problems believing as well.” He said his last sighting was in 2008, and he continues to search, hoping for definitive video or photographic proof.

Matt Mendenhall is the Managing Editor of BirdWatching magazine.

Originally Published February 2, 2012