Some birds mate for life, while others have many partners. Why? In this book, fourteen classic studies are brought together to compare different partnership patterns from ecological and evolutionary perspectives. The subjects have been chosen to include the same species living in different habitats (Sparrowhawks) and at different population densities (Great Tits). There are comparisons between closely related species (Mute Swans and Bewick's Swans; Florida Scrub Jays and Pinyon Jays). The studies span the globe and the behavioural gradient, from Iceland's strictly monogamous Whooper Swans to Australia's sexually promiscuous Splendid Fairy-wrens. In all cases, sexual and social relationships strongly influence a bird's survival and breeding success. Recent research has revealed an astonishing diversity in avian male-female relationships. Social monogamy and sexual fidelity are not necessarily the same thing, and birds have been shown to adopt many variations on this theme.