It’s been dubbed North America’s bird nursery: the sprawling billion-plus-acre boreal forest that spans the continent from Alaska across Canada to Newfoundland and Labrador. Each spring, an estimated 1 billion to 3 billion nesting birds make the long journey north to the boreal forest from wintering grounds throughout the United States and central and South America. Their populations swell during the boreal breeding season, and as many as 3 billion to 5 billion birds of remarkable diversity—from ring-necked ducks to whooping cranes to Cape May warblers to golden eagles—can make the return trip south in the fall.