By Jennifer Kross
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 wings of a mallard and the bill of a pintail, for example, or the tail of a wigeon and the profile of a wood duck. Known as hybrids, these birds result from the mating
of two different species.
Hybridization, or crossbreeding, occurs when an individual of one species enters the geographic or ecological space of another species, and two individuals mate and produce offspring. Hybridization sometimes results from a mixed-species pairing. A male mallard, for example, will pair with a female black duck and prevent other male black ducks from pairing with that female. 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. Mallards and wood ducks in particular have demonstrated the capability of hybridizing with a surprisingly wide range of other species. However, most waterfowl hybrid offspring are infertile.
In North America, one of the most common wild hybrids results from mallard/pintail breeding. Mallards also commonly crossbreed with black ducks, wigeon, shovelers, cinnamon teal, green-winged teal, and gadwalls. In recent years, hybridization between the closely related Eurasian and American wigeons 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.
Many waterfowl hybrids may be unable to attract mates because they are not recognized by individuals of either parent species as their own kind. 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. 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 is occurring within the historical range of the American black duck. Agriculture and forestry practices have altered much of the black duck’s original breeding habitat in the northeastern United States and eastern Canada. This alteration has allowed mallards to expand their range eastward, leading to more interaction with black ducks and increasing opportunities for hybridization.
Changes to black duck migration and wintering habitat have also fostered encroachment by mallards. Forests that once separated these species have been cleared, giving mallards more opportunities to interact with black ducks during the nonbreeding season. Interspecies interactions on the wintering grounds are important because this is when waterfowl form pair bonds for the upcoming breeding season. This interaction could lead to mixed-species pairing and contribute to the hybridization problem.
Many questions exist regarding the degree of threat hybridization with mallards poses to the black duck. Currently, mallards and black ducks coexist in areas of the Atlantic Flyway, and the greatest threat to black duck survival is likely the loss of suitable nesting and wintering habitat.
Introduced mallards have also caused severe hybridization problems and threaten the genetic purity of resident species elsewhere around the world. Island and peninsular populations like the Hawaiian duck and the Florida mottled duck are especially susceptible to the effects of hybridization with mallards. These small, isolated populations of waterfowl are closely related to the mallard, and hybrid offspring are fertile and readily pair with either parent species.
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, mallards are exotic, and today hybridization with mallards is the most significant threat to the Hawaiian duck. Currently, only about 2,200 genetically pure Hawaiian ducks remain, and 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 seven 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 possession and 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.