Feral and wild mallards might be collapsing into a hybrid swarm.
More than a year ago, I wrote a blog post about hybridization between mallards (Anas platyrhynchos) and American Black Ducks (A. rubripes). A study in the journal Ecology and Evolution found low levels of gene flow between these species, but also uncovered some interesting patterns within the mallards populations. The researchers could discriminate between two main mallard populations, which they referred to as western and non-western mallards. Interestingly, the non-western birds, which occur mainly east of the Mississippi River, carry the mitochondrial haplotype A. This genetic variant originates from the Old World and could have reached North America in a number of ways. I ended the blog post with one possibility.
The most likely source for this haplotype is feral ducks, which have been (and are still being) released in North America. The game-farm birds are originally from Eurasia and thus carry haplotype A. Based on these patterns, the researchers propose the following scenario: hybrids tend to backcross with mallards and these backcrosses consequently interbreed with feral ducks. But samples from game-farm birds are needed to confirm this hypothesis. There will definitely be more studies on this system. So keep an eye on the Avian Hybrids Project.
Recently, my prediction came true. Philip Lavretsky and his colleagues sampled feral mallards and compared their genetic make-up with the wild populations. Their findings appeared in the journal Molecular Ecology.
I will not keep you in suspense. The new study confirmed the suspicion that wild mallards acquired haplotype A from feral birds. A haplotype network – which shows the relationships between different mitochondrial variants – clearly shows several wild mallards cluster with the game-farm mallards. More detailed analyses, based on genomic data, corroborated these patterns.
The researchers state that this is “compelling evidence that the presence of the predominant OW [Old World] A mtDNA haplotypes in North America is instead largely the result of a century of game-farm stocking practices.” Indeed, game managers have yearly released more than 500,000 captive-bred ducks on the eastern coast since the 1920s. The numbers have decreased in recent years, but there are still about 210,000 feral mallards being released annually. Moreover, the conversion of boreal forests into open, prairie-like habitat, has allowed mallards to naturally expand eastward since the 1950s. Together, these processes have brought wild and feral mallards into contact, resulting in high levels of hybridization and potentially the formation of a hybrid swarm.
Increased genetic divergence
The hybrid dynamics between wild and feral mallards have important implications for understanding the genetic differences between mallards and black ducks. Although there is frequent hybridization between mallards and black ducks, this does not seem to result in high levels of gene flow. Which reproductive isolation mechanisms are preventing gene flow between these species remains to be determined, but it does lead to increased genetic divergence between mallards and black ducks. This genetic differentiation might be accelerated by hybridization between wild and feral mallards.
In a recent review paper on multispecies hybridization in birds, I wrote that “introgression between certain species might contribute to increased divergence and reproductive isolation between those species and other related species.” The three duck populations in the current study might be a nice example of this idea. Gene flow from feral into wild mallards changes the genetic make-up of the wild populations, resulting in increased genetic differentiation between wild mallards and black ducks.
Lavretsky, P., McInerney, N. R., Mohl, J. E., Brown, J. I., James, H. F., McCracken, K. G., & Fleischer, R. C. (2020). Assessing changes in genomic divergence following a century of human‐mediated secondary contact among wild and captive‐bred ducks. Molecular Ecology, 29(3), 578-595.
This paper has been added to the Anseriformes page.