Hybridization, Introgression, and Morphometric Differentiation between Mallard (Anas platyrhynchos) and Grey Duck (Anas superciliosa) in Otago, New Zealand

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Abstract
Small numbers of Mallard (Anas platyrhynchos) were introduced into New Zealand from Great Britain and North America over 100 years ago. Both sexes have undergone differentiation in size and plumage characters as a consequence of hybridization with the indigenous Grey Duck (A. superciliosa). Pure forms of both species, as documented by early descriptions, appear to be disappearing, particularly the Grey Duck. In Otago, the Mallard and Grey Duck are introgressively hybridizing, and the majority of intergrades are Mallard-like in appearance. Separation of Mallards, hybrid birds, and Grey Ducks was achieved using a stepwise cluster analysis of mensural characters. The observed morphometric differentiation was not continuous throughout the Otago population, and a total of 5 discontinuities (groups of morphologically similar birds) were distinguished: male Mallards, female Mallards, Grey Ducks, and 2 groups of hybrid birds. The use of bivariate scattergrams of skull length and eye-lens weight provided the most useful illustration of the distinctions among Mallard, hybrid, and Grey Duck populations. As a consequence of hybridization, two morphologically distinct hybrid populations have been produced: one resembles the Grey Duck and the other the Mallard. This situation is discussed in relation to the two hybrid forms of the Marianas Mallard (A. oustaleti). The Mallard was so successful in newly created agricultural habitat that by 1958 this species constituted 53% of the Mallard-Grey Duck population. Prior to 1958 the observed hybrid frequency was less than 3% and the reduction in the proportion of the Grey Duck most probably was the result of habitat reduction. In 1977 the Mallard comprised 82% of the Mallard-Grey Duck population, and there was concern over the number of pure Grey Ducks remaining in Otago. By 1981-1982 levels of hybridization, based on plumage analysis, had reached 51%, and the proportion of pure Grey Ducks had dropped to 4.5%, which is below the level suggested for the maintenance of a species. In the absence of reproductive isolation or antihybridization mechanisms between these two species, the Mallard and hybrid populations represent a potential threat to the conservation of the New Zealand Grey Duck.