Oystercatchers are usually socially monogamous, with one male and one female sharing a territory and raising chicks together. But field research has shown that their mating system can be more flexible. A study published in "Nature" in 1998 by Dik Heg and Rob van Treuren described cases where two females and one male form a stable breeding trio.

In these trios, all three birds may bond socially and take part in nesting and chick care. Both females can mate with the male, and female to female sexual behavior has also been observed. Rather than competing until one is excluded, the females sometimes cooperate, which can increase the chances that at least some chicks survive. These bisexual trios show that oystercatcher relationships are not always limited to simple male female pairs, but can involve more complex and flexible social arrangements.

stickers are about 10cm big