This book describes the results of a long-term study of the ecology, evolutionary genetics and sociobiology of a seabird, the Arctic Skua. This species is polymorphic: the birds show one of three, genetically different forms of plumage - pale, intermediate and dark. The forms vary in frequency from predominantly pale in the north to dark in the south. The study was undertaken with the aim of explaining how natural and sexual selection act to maintain all three forms of skua in its populations. The results show that natural selection for pale is balanced by sexual selection for intermediate and dark. Models derived from Darwin's theories of female choice and sexual selection in monogamous birds fit the breeding data of the Arctic Skua. Darwibn's views on sexual selection are fully confirmed. The study produced original data on breeding ecology, demography, population regulation, sexual behaviour and territoriality. A chapter on feeding ecology is also included in the book, which gives a complete and largely original account of the population ecology and sociobiology of a single species of bird.