Behavioural and molecular evidence for specific status of light and dark morphs of the Herald Petrel Pterodroma heraldica

Abstract

The Herald Petrel Pterodroma heraldica has an extensive subtropical breeding range across the width of the Pacific Ocean. It occurs in dark and light morphs, and, at most breeding colonies, the latter is more numerous. However, on Henderson Island, one of the Pitcairn Islands of the Central Pacific, the dark morph is more numerous. During a field study at Henderson Island, we found evidence of reproductive isolation between light and dark Herald Petrels. The morphs bred and courted assortatively. They also tended to breed in different parts of the island (dark birds nearer the coast) and at slightly different seasons (dark birds mostly in the austral winter, light birds more evenly throughout the year) and uttered different calls. Together, these data strongly suggested that the morphs are distinct species. This conclusion was supported by sequence data from a 307‐base pair region of the cytochrome b gene of the mitochondrial genome. When birds of both morphs from Henderson and light morphs from Ducie Atoll (345 km west of Henderson) were sampled, five haplotypes were restricted to light Herald Petrels: they did not occur in dark birds. In contrast, three haplotypes were restricted to dark Herald Petrels on Henderson Island. Thus, the haplotypes of the two morphs were mutually exclusive, further indicating reproductive isolation between them. We propose that the white‐bellied form should retain the name Herald Petrel P. heraldica, while the dark‐bellied form should be granted specific status as the Henderson Petrel Pterodroma atrata (Mathews 1912). The name atrata derives from a description written by Solander on Cook's first voyage. Molecular data from Round Island Petrels Pterodroma arminjoniana confirmed the close relationship between this form and heraldica. However, arminjoniana and atrata did not share sequences for the cytochrome b region studied.