National Geographic: Throughout the year, birders look forward to changing seasons and avian scenes as they explore woods, grasslands, and wetlands: the spectacle of spring migration, the songs of breeding birds, the autumn southward flight of wintering species from northern nesting grounds. Increasingly, though, both casual bird-watchers and ornithologists note a steady decline in numbersnot just of endangered species, but also of common birds not usually considered to be at risk. Study after study, survey after survey...