Since 1500, more than 150 bird species have disappeared from the world, including the much lamented dodo. This ground bird disappeared from its island home before Carl Linnaeus, the father of scientific taxonomy, even described it in the 18th century. Given that many of the nearly 10,000 known bird species have only recently been described--including those only available from remains like the dodo--some biologists suggest that current extinction rates have been seriously underestimated and will rise rapidly in the coming century.

Stuart Pimm of Duke University and his colleagues analyzed current estimates of bird extinction rates. Out of 9,975 known bird species, 154 have disappeared, or roughly 1.3 percent. Extrapolated, this yields an estimate of 26 extinctions per million bird species every year. Based on fossil records, scientists estimate that normal extinction rates average just one lost animal for every million species per year.

But the researchers argue that the estimate of human impact on birds fails to account for new species identified from remains and those species likely to be extinct but not yet declared so in order to continue conservation efforts. Such efforts have had a demonstrable impact, dropping the extinction rate by half over the course of the last century. "Were it not for conservation actions, there would have been these 25 prevented extinctions plus the actual 20 extinctions in the wild over the last 30 years," the scientists write in a paper published online this week in Proceedings of the National Academy of Sciences.

Conservation, however, will not be enough to save many of the bird species most at risk. Habitat destruction partnered with climate change, which particularly impacts those birds lingering at the edges of their ranges, will eventually doom 1,700 species of birds, boosting extinction rates to roughly 1,000 out of one million species every year. "Millions of people are fond of birds," the authors note. "Whether fondness will prove sufficient to protect the thousand or more species threatened with habitat loss across tropical forests remains to be seen. Even if it does, this fondness for birds is not likely to protect completely the remainder of biodiversity."