Island differentiation of New Zealand’s extinct mergansers (Anatidae: Mergini), with description of a new species from Chatham Island

Abstract

Measurements of major bones from three island populations (New Zealand
mainland, Auckland Islands, Chatham Island) of extinct mergansers presently known as Mergus australis indicate that birds from Chatham Island differed in size and proportion from those at Auckland Islands. The Chatham Island specimens were smaller overall, with a shorter skull, relatively shorter premaxilla, smaller sternum and keel, relatively shorter wing bones and a narrower pelvis. These differences support its taxonomic recognition as the new species, Mergus milleneri, described in this paper. Determining reliably the magnitude of the size differences of major bones between mergansers from mainland New Zealand and Auckland Islands or Chatham Island was not possible due to inadequate samples of these sexually dimorphic taxa. However, mainland mergansers may also have been smaller than Auckland Islands birds. Based on these measurements, Holocene fossils of mergansers on New Zealand’s mainland therefore are considered to be Mergus sp. indeterminate and we recommend that Mergus australis henceforth be called the Auckland Islands
Merganser.