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...