Waders (Charadrii) provide biologists with an astonishing variety of mating systems to study(1). Male and female birds establish breeding units in which behaviour varies from monogamy, polygyny, polyandry, double clutching, lekking and serial monogamy to sex role reversal, and many mixed mating systems exist(1). This diversity is currently explained by the costs and benefits of males and females either cooperating or defecting during breeding attempts(2,3). The oystercatcher (Haematopus ostralegus) is a typically monogamous species: removal experiments show that both parents are needed to raise chicks to fledgings(4-6). However, occasional polygyny has also been reported(7). Here we describe polygynous oystercatcher trios and the reproductive consequences of such polygyny. There is a 'classical' form of polygyny (two female territories within the male territory), but oystercatchers also show a remarkable variant, accompanied by female-female cooperation, female-female copulations and joint nesting.