Polyandry in which one female forms either simultaneous or sequential pair bonds with more than one male is a rare vertebrate mateship system. Prior to 1972 only two cases were known with certainty among birds. All other reports of polyandry were based on inadequate information. Recently, polyandry has been demonstrated in several additional species. Surprisingly, they all belong to the avian order Charadriiformes. Comparisons of the existing polyandrous systems suggest that they can be arranged in a sequence that leads through a series of relatively minor differences from monogamy to simultaneous polyandry.