This book focuses on the current understanding of a set of topics in ecological and environmental physiology that are of particular interest to ornithologists, but which also have broad biological relevance. The introductory chapter covers the basic body plan of birds and their still-enigmatic evolutionary history. The focus then shifts to a consideration of the essential components of that most fundamental of avian attributes: the ability to fly. The emphasis is on feather evolution and development, flight energetics and aerodynamics, migration, and as a counterpoint, the curious secondary evolution of flightlessness that has occurred in several lineages. This sets the stage for subsequent chapters, which present specific physiological topics within a strongly ecological and environmental framework. Chapter 2 covers gas exchange and thermal and osmotic balance, together with the central role of body size. Chapter 3 addresses ‘classical’ life history parameters — male and female reproductive costs, parental care and investment in offspring, and fecundity versus longevity tradeoffs — from an eco-physiological perspective. Chapter 4 offers a comprehensive analysis of feeding and digestive physiology, adaptations to challenging environments (high altitude, deserts, marine habitats, cold), developmental adaptations, and neural specializations (notably those important in foraging, long-distance navigation, and song production). Throughout the book, classical studies are integrated with the latest research findings. Numerous important and intriguing questions await further work, and the book concludes with a discussion of research methods and approaches — emphasizing cutting-edge technology — and a final chapter on future directions that should help point the way forward for both young and senior scientists.