By Philip Lavretsky, Ph.D.
While it might be hard to imagine today, mallards were once rarely encountered throughout much of the Atlantic Flyway. That began to change during the 20th century, as the expansion of agriculture in eastern Canada opened the Boreal forest, creating ideal habitat for pioneering mallards from the west. To the south, in the eastern United States, government agencies and private citizens worked for decades to establish a huntable mallard population through the large-scale release of game-farm birds. Mallard numbers grew exponentially in the east, and by the 1960s, the greenhead had become the most abundant duck in the Atlantic Flyway.
Unfortunately, the growth of eastern mallard populations coincided with comparable decreases in the abundance of American black ducks, which declined by more than 50 percent between the 1950s and 1980s. This decline may have resulted from intense competition with mallards for habitat, food resources, and mates. While black duck numbers appear to have stabilized in recent decades, concerns have remained about the future of the species, especially due to persistently high rates of hybridization with mallards.
During the past decade, my colleagues and I conducted research using the latest genetic techniques to understand the extent to which hybridization has affected the genetic integrity of black ducks and mallards in eastern North America. We published our work in the scientific journal Ecology and Evolution in 2019. Using hundreds of samples at the landscape level, coupled with thousands of genetic markers, we discovered a hybridization rate of 25 percent among birds identified as black ducks, meaning roughly a quarter of our samples had mallard DNA. Much to our surprise, however, the remaining 75 percent were pure black ducks. How could that be possible? Our research suggests that mallard−black duck hybrids rarely breed with black ducks, resulting in little gene flow back into the parental black duck population. That is great news for this iconic species, which will likely continue to grace our skies as long as sufficient habitat remains on the landscape.
Perhaps even more interesting, however, were the two unique genetic signatures we found in our wild-mallard sample set. Specifically, whereas pure black ducks consisted of a single genetic stock across their range, mallards showed very distinct genetic differences moving from east to west. Mallards west of the Mississippi all had a distinct "western" genetic signature, while those farther east showed varying degrees of intermixing with birds sharing a novel "non-western" genetic signature. That was the first real clue that something very strange was occurring among eastern mallards.
In addition to questions about the genetic history of mallards and black ducks, we set out to test the hypothesis that the secondary source of mallard DNA present in North America was the result of game-farm mallard stocking practices in the eastern United States. My coauthors, Dr. Helen James, research zoologist and curator of birds at the National Museum of Natural History, and Dr. Robert Fleischer, head of the Smithsonian Conservation Biology Institute Center for Conservation Genomics, and I analyzed DNA samples from historical (1860−1915) museum specimens that were collected before widespread game-farm mallard releases began. These findings were published in the scientific journal Molecular Ecology in 2020. In short, comparing the DNA makeup of today's mallard populations to the museum specimens confirmed that western mallards remain genetically identical to those that lived on this continent 150 years ago, while the genetics of eastern mallards are much different.
When we analyzed the DNA of present-day game-farm mallards from New Jersey and Kentucky, we determined that these birds were of the same genetic stock, and that the source was likely of Eurasian and not North American descent. Our analysis confirmed historical accounts that many game-farm mallards were imported from abroad when attempts to breed wild North American mallards and black ducks failed. These nonnative birds have been propagated—and released into the wild in large numbers—in the eastern United States over the last century.
It's important to note that game-farm mallards should not be confused with "park ducks," such as Khaki Campbells, which are even more remotely related to wild mallards than game-farm birds. We found no evidence of these birds' DNA in the mallard samples analyzed in our study.
But Eurasian game-farm mallards have been subject to human domestication for hundreds to thousands of generations. Strictly looking at the DNA, game-farm mallards have half the genetic diversity of their wild counterparts. In fact, they are 10 times more distantly related to our native western mallards than are black ducks, Mexican ducks, and mottled ducks. Moreover, game-farm mallards weigh 300–400 grams (approximately 10−14 ounces) less than wild birds on average. Their bills also have structural differences likely derived from a diet of domestic grains. Other differences are apparent in their breeding behavior. Males are known to be overly aggressive, while females have a prolonged breeding period, in which they often produce excessive numbers of eggs and show poor nest vigilance compared to wild mallards. These traits make game-farm mallards easier to propagate in domestic settings but are clearly not beneficial in the wild. Together, these differences demonstrate that game-farm mallards only appear to be "wild" on the outside, when they are in fact a genetically modified species.
Nevertheless, our research confirmed that game-farm mallards are capable of not only surviving but also successfully breeding in the wild. In fact, 92 percent of the mallards that we analyzed in the Atlantic Flyway had a substantial amount of game-farm DNA. Our work also revealed that unlike wild mallards, black ducks resist the temptation to interbreed with game-farm birds. In general, these results were in stark contrast to what we expected when we began our research. Instead of the black duck being faced with genetic extinction, it is the native North American mallard that may be threatened by hybridization with game-farm birds of Eurasian domestic descent.
Dr. Philip Lavretsky is an assistant professor of biological science at the University of Texas at El Paso.
FULL TEXT To read the research papers by Dr. Phil Lavretsky and his coauthors that were cited in this article, visit the DU website at ducks.org/duckdna.
ARE WILD MALLARD POPULATIONS AT RISK?
Research published in the scientific journals Ecology and Evolution (2019) and Molecular Ecology (2020) revealed that North America now has two mallard populations: a large wild population of western mallards and a second, eastern population composed of game-farm mallards and hybrids with varying proportions of wild and domestic DNA. Could maladaptive qualities derived from game-farm birds be the cause of declining mallard populations in the Atlantic Flyway? That's an intriguing question, and research is currently under way to further study this hypothesis.
But hybridization between wild and game-farm mallards isn't confined to the Atlantic Flyway. Almost half the birds sampled from the Mississippi Flyway carried domestic mallard DNA, and that trend is increasing. Game-farm genetics are now trickling into the midcontinent mallard population that largely breeds in the Prairie Pothole Region. The sheer number of prairie-breeding mallards may be able to "absorb" the potentially negative effects of hybridization with mallards with domestic DNA, but more research is clearly needed to determine the possible threats game-farm birds pose to wild mallard populations.
Report: Assessing changes in genomic divergence following a century of human-mediated secondary contact among wild and captive-bred ducks
Report: Identifying hybrids and the genomics of hybridization: Mallards & American black ducks of Eastern North America