Sperm competition is the competition between sperm from two or more males to fertilize the egges of a single female during one reproductive cycle. Recently many ornithologists have focused on this theme, and are studying many bird species by using biochemical methods (e. g. DNA fingerprinting). I review almost all of the important literature on avian sperm competition of the last decade, and discuss the evolution of social behaviour of both sexes through sperm competition. Sperm competition is an essential process of intrasexual selection which influences not only the characteristics of reproductive organ and mating behaviour, but also the mating system, social organization and life history strategy of birds. It is a co-evolutionary process between both sexes.