Abstract
Six cases of bigamous trios in the Black-winged Stilt Himantopus himantopus himantopus were studied during 1997-2006. One cause for the occurrence of bigamous trios is that postbreeding groups composed of a parent and a female offspring of the previous year, or that of a family and an unrelated female, have not dissolved by the next breeding season. Under conditions of a female-biased sex ratio in the non-breeding season, some females may chose bigamous trios as one of three mating options. In one trio, one female left the trio just before egg-laying, after they began to defend the nesting territory. Two bigamous trios made a joint nest, respectively. One bigamous trio made two simultaneous nests. One bigamous trio made a joint nest in one year and two simultaneous nests the following year. One bigamous trio made three nests successively.
