Birds show bewildering diversity in their life histories, mating systems and risk of extinction. Why do albatrosses delay reproduction for the first 12 years of their life while zebra finches breed in their first year? Why are fairy-wrens so sexually promiscuous while swans show lifelong monogamy? Why are over a quarter of parrot species threatened with global extinction while woodpeckers and cuckoos remain secure? Some of these topics are classic problems in natural and sexual selection, while others have arisen in the last decade, such as variation in genetic mating systems or extinction risk. Birds offer a unique opportunity for investigating these questions because they are exceptionally well-studied in the wild. By employing phylogenetic comparative methods and a database of up to 3,000 species, the authors identify the ecological and evolutionary basis of many of these intriguing questions. They also highlight remaining puzzles and identify a series of challenges for future investigation. This is the most comprehensive reappraisal of avian diversity since David Lack's classic "Ecological Adaptions for Breeding in Birds". It is also the most extensive application of modern comparative methods yet undertaken. This novel approach demonstrates how an evolutionary perspective can reveal the general ecological processes that underpin contemporary avian diversity on a global scale.