Presented by Joshua I. Brown - Email: firstname.lastname@example.orgA sub-species of the Pacific Black Duck (Anas superciliosa), the New Zealand Grey Duck (A. superciliosa superciliosa) is endemic to New Zealand (NZ). The closely related Mallard (A. platyrhynchos), was introduced to NZ in the mid-1860s, although sustainable breeding populations were not established until the release of 25,000 additional Mallards from 1940-1960. Previous research suggested wide-spread hybridization resulted in only ~5% of pure Grey Ducks remaining throughout NZ and that the population may now be a hybrid swarm. Here, we use a landscape level approach to determine whether hybridization between Mallards and Grey Ducks has indeed resulted in a hybrid swarm (i.e., various generations of hybrids and backcrosses), and to identify any geographic or biological barriers that limit hybridization. During 2014-2018 we collected samples from 673 Mallards, Grey Ducks, and putative hybrids from the North (N = 378) and South (N = 295) Islands of New Zealand. Using ddRAD-seq techniques to sequence ~3,500 nuclear loci, we report that pure Grey Ducks are strongly genetically structured from Mallards (ST = 0.085). However, only 5% and 10% of samples were identified as pure Grey Duck or Mallard, respectively. The remaining 85% of samples comprised a variety of hybrids forms (hybrid swarm) that largely resulted from backcrossing into Mallards, suggesting that assortative mating may limit further dilution of the Grey Duck gene pool. In general, hybrid prevalence was highest on the North Island and east of the Southern Alps mountain range in the South Island, which are areas most affected by urban and agricultural development. In contrast, most pure Grey Ducks were concentrated in more remote montane habitat west of the Southern Alps, indicating that the Alps act as a strong geographic barrier to hybridization. Finally, Mallards were genetically differentiated between Islands, suggesting that gene flow is limited by Cook Strait.