Each spring and fall, an estimated 1 billion birds migrate through the Pacific Flyway, which snakes down from Alaska, along the West Coast of the United States and Mexico, and into South America. Species such as the Tree Swallow, Marsh Wren, and Black-throated Gray Warbler all use this migratory corridor as they make their way to their southern overwintering grounds in fall and their northern breeding grounds in spring. Where many of these birds stop along the way and in what numbers, however, has not been thoroughly studied. Now new research reveals what has been long-suspected but never confirmed: California’s Central Valley and the Colorado River Delta are hotspots for North America’s migratory landbirds. The study, published today in Ornithological Applications, shows that tens of millions of birds depend on the regions’ river and wetland habitat on their journey through a largely dry, arid landscape. While the research team had a hunch that the Central...