Abstract
Stilts Himantopus are long-legged waders of the Order Charadriiformes, Family Recurvirostridae. Two species occur in New Zealand, the Black Stilt H. novaezealandiae and the Pied Stilt H. himantopus leucocephalus, each of which has resulted from separate invasions of Himantopus stock from Australia. The early-invading form turned wholly melanic and became morphologically distinct from the ancestral stock in Australia. This early form (the Black Stilt) was widespread and common in New Zealand until late in the nineteenth century, but its taxonomic status and relationships with the Pied Stilt were never satisfactorily established, In the twentieth century the Black Stilt became restricted in numbers and in range, and today it is very rare, being confined mainly to the Upper Waitaki River Valley.
The aim of this study was to examine the present relationships of Pied and Black Stilts, particularly their ecological interactions. Field work was carried out from 1977 to 1980 mainly at Lake Tekapo where the two species overlapped in breeding range and where maximum sample sizes of Black Stilts could be obtained. The hypotheses proposed to account for the decline of the Black strlt were 1) competetive exclusion by the Pied Stilt, 2) introgressive hybridization with the Pied Stilt and 3) the Black Stilt's inability to adapt to man-induced changes in the environment.
To test these hypotheses I carried out studies of habitat utilization, and of feeding and nesting ecology of the two species. The feeding of Black Stilts was closely adapted to riverine habitats, whereas Pied Stilts utilized lentic habitats more heavily. Where individuals of the two species were present at one locality, the Black Stilts were territorially dominant. The breeding success of Black Stilts was significantly less than that of Pied Stilts, owing to the susceptibility of the former to predation by introduced mammals. Black Stilts nested on dry stream banks where the intensity of predation was very high. In addition, they were solitary nesters, had ineffective distraction displays, had a long fledgling period, and had an early start and often a late end to the breeding season, all of which increased their vulnerability. Pied Stilts exhibited opposite features, including nesting in very wet areas, particularly swamps, where reproductive success was high.
The two species are not reproductively isolated, but hybridize, despite positive assortative mating. Hybrids are fertile and vigorous. Through introgressive hybridization the Pied Stilt population has gained much new genetic material, to the extent that its members are subspecifically distinct from Pied Stilts in Australia.
The decline of the Black Stilt has resulted from its inability to adapt to man-induced changes (particularly the introduction of predators), coupled with introgressive hybridization with the Pied Stilt. Without continued active management, particularly predator exclusion, the population of Black Stilts will decline further, and those remaining will eventually be absorbed into the large Pied Stilt population.