New Zealand has experienced two invasions of stilts, the first giving rise to the endemic Black Stilt (Himantopus novaezealandiae) and the second being that of the Pied Stilt (H. himantopus leucocephalus). The geographical separation of these forms was of insufficient duration for reproductive isolation to become complete, and introgressive hybridisation has occurred. Hybrids are usually intermediate in plumage and morphology between their parents and are distinguishable from immature Black Stilts. There was no evidence of hybrid infertility or lack of vigour. Through hybridisation, the Pied Stilt has become distinguishable from the Australian population of Pied Stilts by several characteristics, including shorter tarsus, longer tail, and variable plumage markings. Selective mating and a different wintering area have helped keep the small remnant population of Black Stilts from being absorbed into the much larger Pied Stilt population. On the basis of aspects of its morphology, ecology and behaviour, the Black Stilt merits its status as a full species.