A close relative of the Mallard (Anas platyrhynchos) endangered Hawaiian Duck is 1 of only 3 endemic species of waterfowl in the Hawaiian Islands today. Known locally as Koloa Maoli (native duck) or simply Koloa, this species historically inhabited the main Hawaiian Islands, except the dry islands of Kaho‘olawe and Läna‘i. In the mid-1800s, it was considered common and was hunted for sport. By 1900 and again in the early 1920s, however, several ornithologists began to note the species' decline (Perkins 1903, Munro 1939a), and it was extirpated from all islands but O‘ahu, Kaua‘i, and Ni‘ihau by the 1950s. By 1962, it was restricted to the latter 2 islands, and population estimates at that time indicated fewer than 500 individuals.
The decline of the Hawaiian Duck is directly related to the destruction of key wetland habitats in the Hawaiian Islands, particularly Waikïkï, Ka‘elepulu, Köloa Swamp, and Kawai nui marshes on O‘ahu and the Mana wetlands on Kaua‘i. On Hawai‘i, Maui, and Moloka‘i, this species was never abundant in historic times, owing to limited habitat. In addition to habitat loss, predation from introduced mammals dealt a severe blow to the species. Sport hunting continued well into the first third of the twentieth century and has been mentioned as a factor in the species' decline (Swedberg 1967). In addition, the Hawaiian Duck is confronted with the modern threat of hybridization with feral Mallards (Anas platyrhynchos), the single most important threat facing its continued existence. Hybrid swarms now exist on O‘ahu and Maui, and hybrid birds have recently been documented from Hawai‘i and Kaua‘i (AEJr, KJU). By 2000, populations of genetically pure Hawaiian Duck existed only on Kaua‘i and the upper elevations of Hawai‘i Island (the latter a result of reintroduction through captive propagation and release). The population is estimated to be 2,000 birds, but this is at best a guess, and there remains no doubt that it is among the rarest of the world's birds and its continued existence remains in doubt.
Hawaiian Duck literature can be grouped into 3 categories: (1) historical accounts of early naturalists from European contact to the mid-1900s (Henshaw 1902a; Perkins 1903; Munro Munro 1939a, Munro 1960); (2) Pittman-Robertson (Federal Aid to Wildlife Restoration Act) field studies on waterbird ecology, life history, and propagation conducted by the Hawai‘i State Division of Forestry and Wildlife (herein referred to as DOFAW); and (3) recent research on genetics regarding phylogeny, systematics, and introgression due to Mallard hybridization (Griffin and Browne 1990, Rhymer 2001). The Hawaiian Duck has been included in works on waterbird distribution, habitat (Shallenberger 1977a), population histories (Banko 1987a), management strategies (Chang 1990), status, population-trend analysis (Engilis and Pratt 1993), and endangered-species recovery (U.S. Fish and Wildlife Service 1999c). Only one comprehensive study of life history exists (Swedberg 1967). However, for an anatid that was first documented more than 220 years ago, catalogued as a unique Hawaiian species 120 years ago, and listed as federally endangered in 1978, information is scant at best. It remains one of the more poorly known of Hawaiian birds and of the world's waterfowl, and much of its life history is first reported in this account.