Some of the most unusual and taxonomically puzzling taxa of dabbling ducks are endemic to remote oceanic islands. The Laysan Duck, endemic to the Hawaiian Islands, has within historic times been restricted to tiny, remote Laysan Island near the northwestern end of the archipelago. As a result, this species has perhaps the most restricted distribution of any living duck, although recent subfossil evidence indicates it once inhabited many of the main Hawaiian Islands.
Laysan Island (Figure 1 and Figure 2), about 370 ha in total area, including unvegetated areas and the central hypersaline lake upon which the species depends for much of its invertebrate diet, seems to have a carrying capacity of about 500 Laysan Ducks. Relatively long-lived with a low reproductive rate, this species can and does fly but is much more prone to walk or run, especially in pursuit of its main prey, brine flies (Neoscatella sexnotata, Ephydridae). The anatomy of the Laysan Duck reflects these habits.
Between 1905 and 1925, the species came close to extinction on Laysan Island, largely because of the intentional introduction of European rabbits (Oryctolagus cuniculus), which almost completely destroyed the island's endemic vegetation. After the rabbits starved around 1924, the island revegetated and the species recovered, although 3 of 4 other endemic land birds suffered extinction.
This severe population decline further reduced the genetic diversity of the wild population. Another, less serious, population decline in 1993-1994 apparently was due to a drought, the related decline of brine flies, and a suspected parasite outbreak.
The Laysan Duck will likely remain on the Federal Endangered Species List owing to its small population, restricted distribution, and vulnerable island habitat.
Formerly called the Laysan Teal because of its small size, the Laysan Duck is a dull, dark brown dabbling duck with a prominent white eye-ring; the sexes look much alike. White feathering on the head and neck (leucisticism) is common in this species and, as for certain other island species, may indicate reduced genetic diversity of the population. Certain plumage, bill color, and behavioral characteristics suggest a Mallard (Anas platyrhynchos) ancestry, but genetic evidence now suggests a different evolutionary history.