In early April, as Owen and his sister search the hickories, oaks, and dogwoods for returning birds, a huge group of birds leaves the misty mountain slopes of the Yucatan peninsula for the 600-mile flight across the Gulf of Mexico to their summer nesting grounds. One of them is a Cerulean warbler. He will lose more than half his body weight even if the journey goes well. Aloft over the vast ocean, the birds encourage each other with squeaky chirps...