Without the influence of humans, the expected extinction rate for birds would be roughly one species per century, according to Stuart Pimm, professor of conservation ecology at Duke University's Nicholas School of the Environment and Earth Sciences, who is one of the report's principal authors.
"What our study does, for the first time, is provide a well-justified and careful estimate of how much faster bird species are going extinct now than they did before humans began altering their environments," said Pimm, whose research group pioneered the approach of estimating extinction rates on a per-year basis.
"Extinction rates for birds are hugely important, because people really care about birds," he said. "People enjoy them, and bird watching is a big industry. So we know the rates of bird extinctions better than the rates for other groups of species."
"Habitat destruction, selective hunting, invasive alien species and global warming are all affecting natural populations of plants and animals adversely," added Peter Raven, president of the Missouri Botanical Garden, who is co-principal author of the report and a longtime collaborator with Pimm.
The report will appear in the online edition of the Proceedings of the National Academy of Sciences during the week of July 3-7, 2006. Other authors are Alan Peterson, a physician in Walla Walla, Wash., and Paul Ehrlich and Cagan Sekercioglu, conservation biologists at Stanford University.
The researchers calculated that since 1500 -- the beginning of the major period when Europeans began exploring and colonizing large areas of the globe -- birds have been going extinct at a rate of about one species per year, or 100 times faster than the natural rate.
And the rate has been faster in recent times. "Increasing human impacts accelerated the rate of extinction in the 20th century over that in the 19th," the report said. "The predominant cause of species loss is habitat destruction."
These findings do not mean Europeans have caused all of the extinctions of birds over the course of time, the researchers said. "Europe's exploration of the rest of the world merely continued to extinguish species at rates similar to those caused by the earlier Polynesian expansion across the Pacific," they said in the report.
The new assessment considerably exceeds previous scientific estimates that 154 bird types disappeared during that past 500 years, according to the researchers.
One factor contributing to such large differences in estimates is that "more than half of the known species of birds were not discovered until after 1850, an important point that previous estimates of extinction rates have failed to take into account," Raven said. "One can't register a bird as extinct if it was not known to exist in the first place."
According to Pimm, as recently as 1815 scientists were aware of only about 5 percent of the world's birds. "The reality is that scientists did not know about most remaining bird species until about 1845 or 1850," he said.
The new report is not all bleak, Pimm said. "The good news in this report is that conservation efforts are reducing extinction rates to about one bird species every three or four years," he said, but he added that even this improved rate "is still unacceptable."
Of the 9,775 known species of birds, "an estimated additional 25 would have gone extinct during the past 30 years if it were not for human intervention," Raven said.
Despite conservation efforts, "some 1,200 more species are likely to disappear during the 21st century," he warned. "An equal number are so rare that they will need special protection or likely will go extinct, too."
The forecast may be even bleaker for other types of animals, the researchers said.
"We do not give the kind of special attention to other groups of organisms that we do to birds, and extinction rates for them are likely to be much higher over the 21st century and beyond," Raven said.
The researchers derived their estimates using a large database of threatened and endangered species compiled by Bird Life International in Cambridge, England. They also used a compilation by report co-author Alan Peterson of the first scientific descriptions of bird species.
"Knowing when species were first described to science turned out to be a hugely important part of this story," Pimm said.
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