Genetic status of New Zealand black stilt (Himantopus novaezelandiae) and impact of hybridisation

The New Zealand black stilt, Himantopus novaezelandiae, is critically endangered, with only 67 "pure" birds remaining, all in the upper Waitaki Basin. Hybridisation with pied stilt, H. leucocephalus, is compounding problems caused by lack of habitat and predation. This report looks at the likely impact and implications of this hybridisation from a genetical perspective. Based on morphological, genetical and phylogenetic data, it is concluded that the two taxa merit recognition as distinct species under most species concepts. With reference to plumage characteristics alone, it appears that females with a degree of hybridity show reduced fitness with respect to the fledging success and survival of their offspring. Simulations of gene flow from pied into black stilts, using a simple single locus two allele model, suggest that reduced fitness in hybrids may be sufficient to counteract low levels of gene flow, preventing genetic swamping of black stilt in the short term at least. A key unknown factor here is the degree of assortative mating under different population sizes and densities. Based on these findings, several management recommendations are made, including: 1) to carry out multivariate analysis of morphological data to determine the best estimators of hybridity; 2) to use these morphological characters as well as plumage to measure hybridity of birds used for captive rearing; 3) to determine the heritability of these traits; and 4) to carry out a more detailed analysis of outbreeding depression using pedigree information. As a general conclusion, although there are genetical threats to the species, the main issues are, as usual, habitat loss and predation, and the associated stochastic problems of small population size.