Every spring, billions of birds sweep north, driven by ancient instincts to return to their breeding grounds. This vast parade often goes unnoticed, except in a few places where these small travellers concentrate in large numbers. One such place is along Lake Erie in northwestern Ohio. There, the peak of spring migration is so spectacular that it attracts bird watchers from around the globe, culminating in one of the world's biggest birding festivals.

Millions of winged migrants pass through the region, some traveling thousands of miles, performing epic feats of endurance and navigating with stunning accuracy. Now climate change threatens to disrupt patterns of migration and the delicate balance between birds, seasons, and habitats. But wind farms - popular as green energy sources - can be disastrous for birds if built in the wrong places. This is a fascinating and urgent study of the complex issues that affect bird migration.