Female-female cooperation in polygynous oystercatchers

The oystercatcher, Haematopus ostralegus, is a monogamous species, and studies show that both parents are required to raise chicks to fledglings. However, polygyny is also occasionally reported. Polygynous oystercatcher trios are described. The classical form of polygyny, two female territories within a male territory, was observed, but other variants were female-female cooperation, female-female copulations and joint nesting.