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.