Love thine enemy?

Oystercatchers are socially and sexually monogamous birds, but a new study reflects another side to their lifestyle. Occasionally, an usurping female will try to break up a pair, and, after fighting with the resident female, she may stay to form a polygynous trio. The male is mated by both females, and they cooperate to defend their territory and raise a brood. Although polygyny is very rare in oystercatchers, the authors suggest that both females benefit in terms of elevated social status.