In pairs, male and female birds appear to work as a team, competing for food, defending nests, and protecting predators, but in fact, because each individual strives to maximize its own reproductive potential, conflicts can occur if one finds a better partner. So while some birds choose one mate for life, others have many partners. In this book, fourteen classic studies of bird behavior are brought together to compare the different partnership patterns from ecological and evolutionary perspectives. Particular attention is paid to the availability of mates, the selection process and the consequences of choosing one mate over another. By comparing between and within species, each chapter outlines the features that influence a pair's reproductive performance and how this interacts with ecological and environmental constraints. Introductory and concluding chapters review the latest thinking on this fascinating subject. The book is aimed at students and researchers in behavioral ecology but can also foster new insights for bird watchers and ornithologists.