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.