FEMALE-FEMALE COOPERATION IN POLYGYNOUS OYSTERCATCHERS

Waders (Charadrii) provide biologists with an astonishing variety of m ating systems to study(1). Male and female birds establish breeding un its in which behaviour varies from monogamy, polygyny, polyandry, doub le clutching, lekking and serial monogamy to sex role reversal, and ma ny mixed mating systems exist(1). This diversity is currently explaine d by the costs and benefits of males and females either cooperating or defecting during breeding attempts(2,3). The oystercatcher (Haematopu s ostralegus) is a typically monogamous species: removal experiments s how that both parents are needed to raise chicks to fledgings(4-6). Ho wever, occasional polygyny has also been reported(7). Here we describe polygynous oystercatcher trios and the reproductive consequences of s uch polygyny. There is a 'classical' form of polygyny (two female terr itories within the male territory), but oystercatchers also show a rem arkable variant, accompanied by female-female cooperation, female-fema le copulations and joint nesting.