By Jennifer Kross and Kai Victor

Updated: January 10, 2024

Nearly every waterfowl season, a hunter brings a photo of a strange duck to a DU biologist and asks, "What kind of duck is this?" Typically, the bird has characteristics of two waterfowl species-the head of a wigeon and the body of a pintail, for example, or the chest of a mallard and the crest of a wood duck. Known as hybrids, these birds result from the mating of two different species.

Hybridization sometimes results from a mixed-species pairing, where a male from one species will establish a pair bond with a female from another species. But hybridization can also result from forced copulation, where a male of one species forcibly mates with a female of another species.

Waterfowl crossbreed more often than any other family of birds. Scientists have recorded more than 400 hybrid combinations among waterfowl species. While many hybrids involve closely related species, some distantly related crosses are known, including muscovy duck × mallard, common eider × common merganser, and American wigeon × redhead.

Mallards and wood ducks in particular have demonstrated the capability of hybridizing with a wide range of other species. In North America, one of the most common wild hybrids results from mallards and northern pintails interbreeding. Mallards also commonly crossbreed with American black ducks, mottled ducks, Mexican ducks, wigeon, northern shovelers, green-winged teal, and gadwalls. In recent years, hybridization between the closely related Eurasian and American wigeon has become more common in Alaska.

In general, hybridization is rare because each waterfowl species has unique characteristics that serve as barriers to interspecies mating. These characteristics include distinct physical attributes, behaviors, life-history requirements, and the unique ecological niche the species occupies. But on the breeding grounds, territories of many waterfowl species overlap, and barriers occasionally break down, presenting opportunities for interspecies mating.

Hybrids often exhibit intermediate physical characteristics and behaviors that render them unable to attract a mate.  Male hybrids, in particular, may not have the attractive plumage or the ability to perform courtship rituals necessary to establish and maintain pair bonds.

Beyond creating interesting-looking ducks, hybridization can potentially lead to the extinction of a species. Introgressive gene flow is what ornithologists call the process by which hybridization causes a species to decline. This process occurs when individuals of two species mate and produce fertile offspring, which then mate with the sensitive parent species and essentially contaminate the pure genes of that species, possibly leading to genetic extinction. Mallards are highly aggressive breeders, and several cases involving mallard hybridization with closely related species present waterfowl biologists with conservation challenges.

One example of mallard hybridization involves the closely related American black duck. Historically, mallards were rare east of the Mississippi River, reducing their likelihood of encountering black ducks. However, due to habitat conversions that allowed wild mallards to expand into the region and deliberate releases of game-farm mallards along the east coast, mallards have increased in abundance across the eastern United States since the 1950s.

Contrary to early fears, scientists have recently determined that introgressive hybridization is not a pressing threat to the genetic existence of black ducks. When hybrids backcross with one of the parent species, the resulting offspring will lose some of their mixed ancestry. After only a few generations of backcrossing with a parent species, the offspring will appear genetically pure. Despite frequent mallard × black duck hybridization, scientists believe the American black duck population remains large enough to absorb the mixed ancestry genes of hybrid offspring and protect the genetic integrity of black ducks.

Island and peninsular populations like the Hawaiian duck and the Florida mottled duck are especially susceptible to the effects of hybridization with mallards. Hybrid offspring are fertile and readily pair with either parent species, decreasing the number of pure birds.

The Hawaiian duck, like many birds in Hawaii, is extremely sensitive to human activity and vulnerable to extinction because of the introduction of nonnative species. In Hawaii, ongoing hybridization with exotic mallards is the most significant threat to the Hawaiian duck. Currently, less than 2,000 genetically pure Hawaiian ducks remain, and given such a small population, the favored management strategy to save this species is to eradicate all mallards and mallard × Hawaiian duck hybrids.

The Florida Fish and Wildlife Conservation Commission (FWC) lists hybridization with feral mallards as an immediate threat to the Florida mottled duck. Historically, mallards only wintered in Florida and migrated north during the breeding season, but released mallards now remain in Florida year-round. These feral birds readily pair and mate with wild mottled ducks. FWC has taken a stand to protect the state's native mottled ducks and has implemented a conservation plan to halt the progression of feral mallard × mottled duck hybridization.

Genetic research is ongoing to determine the extent of hybridization, but current estimates suggest that between 7 and 12 percent of Florida mottled ducks may be hybrids. FWC has established education and public-awareness campaigns to help save the Florida mottled duck, and the release of captive-reared mallards is now prohibited in the state.

The variety of waterfowl species that exist throughout the world is important because each one is uniquely adapted to its environment. Most biologists agree that conservation of waterfowl habitat is the best way to maintain environments suited for each species, preserve diversity, and ensure species survival. However, when a waterfowl species is threatened by hybridization, habitat conservation may not be enough to protect it.

Jennifer Kross is a communications biologist at DU's Great Plains Region. Kai Victor is a graduate student at the University of Delaware.