Genetic variation and management of black stilts, Himantopus novaezealandiae, and pied stilts, H. h. leucocephalus (O. Charadriiformes)

Abstract

The New Zealand genus Himantopus consists of two species, the endangered black stilt, Himantopus novaezealandiae, and the pied stilt Himantopus himantopus leucocephalus, closely related to Himantopus himantopus leucocephalus in Australia. Results from starch gel electrophoretic analyses at 17 genetic loci scored from blood, and 28 genetic loci scored from tissue of 120 specimens were used to assess genetic differentiation across an extensive hybrid zone between the taxa. Eight loci were polymorphic within New Zealand pied stilts, and two loci were polymorphic within Australian pied stilts and black stilts. New Zealand pied stilts had higher H values (H = 0.054, n = 17 loci; H = 0.036, n = 28 loci) than black stilts (H = 0.033, n = 17 loci; H = 0.029 n = 28 loci), and Australian pied stilts (H = 0.033, H= 0.021 respectively). Nei's unbiased genetic distances of D = 0.022 (28 loci) and D = 0.039 (17 loci) between black stilts and Australian pied stilts were within the range commonly reported between non-passerine species. Values of D = 0.006 (28 loci) and D = 0.01 (17 loci) between New Zealand pied stilts and black stilts, and D = 0.004 (28 loci) and D = 0.008 (17 loci) between New Zealand and Australian pied stilts were similar to mean genetic distances reported between avian subspecies and hybridising species. The morphological taxonomy of the New Zealand genus Himantopus was reassessed based on measurements of both live individuals and study skins. Using a hybrid index series for comparison, canonical discrimination of study skins correctly classified Australian pied stilts (94%) and black stilts (79.3%). New Zealand pied stilts formed and intermediate group, and were classified either as black stilts (42.9%) or Australian pied stilts (42.9%). Both electrophoretic and morphological evidence show that New Zealand pied stilts and black stilts are introgressively hybridising species or sub-species. It is proposed that New Zealand pied stilts are hybrids between ancestral populations of H. himantopus in Australia, and H. novaezealandiae in New Zealand. Whether species or sub-species, black stilts and their hybrids are unique to New Zealand. In my opinion, New Zealand pied stilts and Australian pied stilts are conspecific, and black stilts should retain their full specific status. A pedigree should be kept of black stilt pairs and their offspring, and hybrid culling discontinued.