Recent study explored the genetics of this species across the Hawaiian Islands.
The Hawaiian Duck or Koloa (Anas wyvilliana) is a special bird. This species is endemic to the Hawaiian Islands (as you might have guessed) and probably originated about 3000 years ago from a hybridization event between Mallard (A. platyrhynchos) and Laysan Duck (A. laysanensis). In other words, the Hawaiian Duck is a hybrid species (you can read this blog post for more on hybrid bird species). Unfortunately, this unique species is threatened with extinction by extensive hybridization with feral Mallards. An ironic twist for this hybrid species.
Feral Mallards were introduced on the Hawaiian Islands in the 1800s, mainly for food and hunting. During the 1930s and 1940s, several “wild” feral populations were established on different islands. The increasing numbers of Mallards resulted in hybridization with the local Hawaiian Ducks. Genetic analyses confirmed gene flow between these species, raising the fear that the Mallard is genetically swamping the Hawaiian Duck. A recent study in the journal Molecular Ecology assessed the current situation and genetically characterized several island populations.
Caitlin Wells, Philip Lavrestky and their colleagues sampled birds throughout the Hawaiian archipelago and sequenced their DNA using a ddRAD-approach. The genetic patterns that emerged from the analyses painted a dual picture of hope and despair. On the positive side, the island of Kaua‘i (which supports the biggest population of Hawaiian Ducks) contained few hybrids and seemed practically pure.
The lack of hybrids on Kaua‘i can probably be explained by the skewed sex ratio of three males for every one female. Hybridization might occur when females have a difficult time finding a mate from the same species. In the end, they settle for a male from another species (i.e. the desperation hypothesis). This situation does not occur on Kaua‘i because there are plently of Hawaiian males to choose from.
The other islands, however, showed different degrees of hybridization between Hawaiian Ducks Mallards. The first-generation hybrids interbreed with each other or backcross into feral Mallards. The result is a collection of hybrid swarms that blur the line between the parental species. The fitness of effects of this genetic mash-up remain to be investigated. The authors list potential consequences of feral genes in Hawaiian Ducks, including higher growth rates, smaller digestive organs and lower survival rates.
From a conservation point of view, the reseachers provide clear guidelines: “our results suggest that the removal of feral mallards is critical and should be considered a management priority to limit the chance that remaining koloa will be lost to hybridization.” Let’s hope we can save this hybrid species from genetic extinction.
Wells, C. P., Lavretsky, P., Sorenson, M. D., Peters, J. L., DaCosta, J. M., Turnbull, S., Uyehara, K. J., Malachowski, C. P., Dugger, B. D., Eadie, J. M. & Engilis Jr, A. (2019). Persistence of an endangered native duck, feral mallards, and multiple hybrid swarms across the main Hawaiian Islands. Molecular Ecology, 28(24), 5203-5216.
This paper has been added to the Anseriformes page.