Migrating birds today must contend with numerous dangers and challenges that never existed before, from twirling wind turbines and shiny glass buildings in the sky to ever-shrinking amounts of stopover habitat and scores of outdoor cats lying in wait. A report from the Connecticut-based Great Hollow Nature Preserve & Ecological Research Center is now warning of a new, invisible threat to bird migration ─ mercury pollution. The article, recently published online in the journal Ecotoxicology, presents a sobering assessment of the many ways in which global mercury pollution from coal combustion and other human activities threatens to interfere with the ability of birds to successfully migrate, including their ability to navigate, sustain flight for long periods, rapidly refuel during stopovers, and avoid sickness and oxidative stress.