Book excerpt: For centuries biologists have tried to understand the underpinnings of avian migration: where birds go and why, why some migrate and some do not, how they adapt to a changing environment, and how migratory systems evolve. Twenty-five years ago the answers to many of these questions were addressed by a collection of migration experts in Keast and Morton's classic work Migrant Birds in the Neotropics. In 1992, Hagan and Johnston published a follow-up book, Ecology and Conservation of Neotropical Migrant Landbirds. In Birds of Two Worlds Russell Greenberg and Peter Marra bring together the world's experts on avian migration to discuss its ecology and evolution. The contributors move the discussion of migration to a global stage, looking at all avian migration systems and delving deeper into the evolutionary foundations of migratory behavior. Readers interested in the biology, behavior, ecology, and evolution of birds have waited a decade to see a worthy successor to the earlier classics. Birds of Two Worlds will complete the trilogy and become indispensable for ornithologists, evolutionary biologists, serious birders, and public and academic libraries.