BirdLife International, a global alliance of conservation groups, has released its annual assessment of world-wide bird populations, and the news isn't good. 2,000 different species — more than one-fifth of the world's total — are either endangered or threatened with extinction. Humans are the biggest threat, either through destroying bird habitats, or by bringing pests and predators to new areas where they hunt defenseless birds. But humans are also the birds' best hope, if we can figure out ways to preserve these species before they disappear.