"Our study found species ranges in general tend to get overestimated, but that this trend is particularly pronounced for birds that are threatened, rely on specialized diets or have small habitats," said University of California-San Diego Assistant Professor Walter Jetz, lead author of the study. "This suggests that many threatened species of birds may be even rarer than we believe and are in greater danger of going extinct."

The study's co-author, researcher Cagan Sekercioglu of Stanford University, added: "Our findings indicate that the ranges of most vulnerable bird species are experiencing the highest overestimation, thereby painting a rosier picture of their distributions than is actually the case. This suggests that the conservation status of many narrow-ranging, specialized and threatened bird species may be worse than we think."

The research by Jetz, Sekercioglu and James Watson of Britain's Oxford University is to appear in the February issue of the journal Conservation Biology and is now available online at the journal's Web site.