Morphology and Relationships of the Hoary-headed Grebe and the New Zealand Dabchick

Abstract

The Hoary-headed Grebe (Poliocephalus poliocephalus) and the New Zealand Dabchick (P. rufopectus) comprise a super- species without close relatives. The former alone has a distinct winter plumage and is smaller and less intensely pigmented. Soft parts colours, wing patterns, and patterns of downy young are described. Little significant geographic variation was found in either species. As in other grebes, sexual dimorphism is greatest in bill measurements of both species. The greatest mensural difference between the two species is in bill length, but males of the smaller species (poliocephalus) have longer bills than females of the larger species. Pattern of downy young and number of fused thoracic vertebrae support a relationship of Poliocephalus with the genus Rollandia.