Relative risk of extinction of passerine birds on continents and islands

Greater numbers and higher proportions of recent species extinctions have been on islands rather than on continents. In contrast, predictions of massive future extinctions stem from the current clearing of continental, tropical forests1. For instance, since 1600, 97 out of 108 bird extinctions have been on islands2. However, 452 of the total 1,111 species currently considered to be threatened are continental3. Island flora and fauna are uniquely vulnerable to the human introduction of previously absent predators, diseases and other menaces4, whereas species on continents are not so ecologically naive. So could predictions of future continental extinctions based on island histories be exaggerated1? Most threatened species have small geographic ranges5, and the ranges of island species are inevitably smaller than those of continental species. For a given range size, how do the proportions of threatened island and continental species compare? Here we compile the ranges of thepasserine (perching) birds of the Americas. Corrected for range size, continental species are more—not less—likely to be threatened. We use this unexpected vulnerability of continental species with small ranges to produce a map showing where species losses might occur in the long term.