This book reveals the remarkable new history of how passerines diversified and dispersed across the entire world. It also presents and explains the new classification, which reflects the phylogenetic history. The new insights reveal that many of the old evolutionary lineages comprise only a few species that remained in their area of origin or underwent limited dispersal. Only a small number of groups underwent significant proliferation of new species and just five (of 145) passerine families are represented on all continents but Antarctica. Even so, the global variation in species richness generally correlates well with the variation in productivity across different environments. We see how a seemingly constant overall rate of evolution of new species is possible because of rapid proliferation in new ecological niches, including archipelagos, and an extraordinary accumulation of endemic species in certain tropical mountain ranges. In addition to describing the revised evolutionary history of passerine birds, the authors try to identify adaptational changes, including shifts in life history strategies, that underlie major evolutionary expansions. Their aim is to further the development of a unified theory to explain how the prodigious variation of Earth’s biodiversity is generated.