Along the 4,000-mile-long Pacific flyway—one of four main routes in North America for migrating birds—up to six million ducks, geese, and swans wing south every year to find warmth after raising young in the rich habitats of Alaska, Canada, and Siberia. They are joined by millions of shorebirds, songbirds, and seabirds, including the ultimate endurance winner, the arctic tern.

But California's drought has dried up its wetlands. Many insects, fish, and plants are gone. As a result, some migrating birds have died or been depleted of so much energy that they have trouble reproducing. Thousands of ducks and geese, crowded onto parched rivers and marshes, are felled by botulism and cholera, which race through their feeding grounds.

(Long-billed Dowitcher by Andrew Reding)