From Scientific American
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.
Read more at Scientific American