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Abstract: Life history and ecological characteristics of extant mergansers (Tribe Mergini) are summarised and used to infer those likely displayed by the extinct merganser from “mainland” New Zealand (Mergus sp. indeterm.). I speculate this was a river-dwelling species, plausibly a year-round territorial occupant of mid-lower reaches of rivers, whose subadults and non-breeders may have aggregated seasonally on broad lower reaches, including estuaries. Of extant mergansers, its ecology was probably most similar to that of Brazilian merganser (Mergus octosetaceus). Holocene sea-level rise and loss of habitat may have induced changes in social structure of Chatham Island merganser. A plausible life history and ecological template, however speculative, can aid evaluations arising from other sources of evidence e.g. locations of fossils and bone stable isotope chemistry.