The Chatham Island merganser is known only from fossil bones a few thousand years old on Chatham Island and was described formerly only in 2014. It and the extinct Auckland Island merganser are the only mergansers known from Australasia and Oceania. Little is known about the Chatham Island merganser as there are no records of it being seen alive. Evidently it became extinct in prehistoric times from human impacts. The Chatham Island species name honours Dr Philip Millener, former curator of fossil birds at New Zealand’s national museum, who collected most of the material that is known for the species.

Identification

The Chatham Island merganser was smaller than all other Mergus species. Compared with its close relative, the Auckland Island merganser, the Chatham Island species had a shorter skull and beak and larger salt-glands. The Chatham Island bird’s wing, pectoral and pelvic bones were also smaller but its leg bones were a similar size to those of the Auckland Island merganser. Most bones of the Chatham Island merganser were 3-6% smaller than those of the Auckland Island species. Compared with other Mergus species, the Chatham Island and Auckland Island mergansers had a lesser ability to fly but they were not flightless. Male mergansers of all species are a little larger than females. Presuming that the Chatham Island species shared key plumage features with its Auckland Islands cousin, the Chatham Island species was probably a relatively dull-coloured bird with a small head crest.

Distribution and habitat

Fossils of the Chatham Island merganser have been found in natural and archaeological sites only on the main Chatham Island and not on the smaller offshore islands. Suitable habitat on Chatham Island would have included the many small lakes, small slow peat-stained rivers and the dominating barrier-bar Te Whanga Lagoon, which comprises 20% of the island’s area. The Chatham Island merganser’s skeletal proportions suggest that it may have been evolving towards a more sedentary lifestyle.

Population

Fossils suggest that the Chatham Island merganser was once widespread on Chatham Island.

Threats and conservation

The Chatham Island merganser became extinct in prehistoric times, and so no records of live birds exist. A few bones have been found in human food middens, and hunting for food by people is the most likely cause of the species’ demise.

Breeding

Abundant remains of adult Chatham Island mergansers, and some chick and egg remains, were retrieved from a small cave on the shoreline of Te Whanga Lagoon. This indicates that the cave was a popular nesting site for the species. This might have been expected because other merganser species worldwide are hole-nesters and females carry out all incubation duties.

Behaviour and ecology

Other than its nesting behaviour, almost nothing is known of the Chatham Island merganser’s behaviour or ecology. The extensive salt glands on the skull of the species suggest that it spent a considerable time in coastal waters, particularly perhaps the saline waters of Te Whanga Lagoon. All mergansers have tooth-like serrations on their bill margins, used for catching fish.

Food

Mergansers are primarily fish eaters, which they pursue and then catch with their serrated bill-margins.  Nothing specific is known about the food of the Chatham Island species but presumably the diverse freshwater fish fauna would have provided abundant fare. It is likely to have fed on a broad range of fishes, and perhaps aquatic invertebrates, in the large brackish Te Whanga Lagoon and possibly in shallow coastal waters as well.

Websites

Wikipedia

References

Tennyson, A.J.D.; Martinson, P. 2007 (revised edn). Extinct birds of New Zealand. Wellington, Te Papa Press.

Watanabe J. 2017. Quantitative discrimination of flightlessness in fossil Anatidae from skeletal proportions. Auk 134: 672-695.

Williams, M.; Tennyson, A.J.D.; Sim, D. 2014. Island differentiation of New Zealand’s extinct mergansers (Anatidae: Mergini), with description of a new species from Chatham Island. Wildfowl 64: 3-34.

Williams, M.; Tennyson, A.J.D.; Sim, D. 2015. Island differentiation of New Zealand’s extinct mergansers (Anatidae: Mergini), with description of a new species from Chatham Island: correction and addition. Wildlfowl 65: 167-170.

Recommended citation

Tennyson, A.J.D. 2015 [updated 2017]. Chatham Island merganser. In Miskelly, C.M. (ed.) New Zealand Birds Online. www.nzbirdsonline.org.nz