By J. Brian Davis, Ph.D.
Several adjectives can be used to describe the plumage of ducks and geese—beautiful, elegant, exquisite, and spectacular. Each species of waterfowl has unique plumage, which allows individual birds to identify members of their own kind. In addition, many male ducks use their brightly colored feathers to impress mates during courtship displays, and the drab brown plumage of female ducks helps camouflage the birds while they are on the nest.
However, waterfowl do not always have "perfect" plumage. Several factors can cause plumage aberrations in ducks and geese. Some of these odd birds have genetic mutations or are the result of hybridization between different species. Others may be "exotics" that escaped from captivity or somehow reached our shores from other continents. Let's take a closer look at some of the unusual waterfowl that you may encounter in the field this fall.
Albino and Leucistic Birds
Albinism is the most dramatic example of unusual plumage in waterfowl. True albino birds completely lack melanin in their feathers, eyes, and skin due to the absence of the enzyme tyrosinase. Like their feathers, the eyes of albino birds are colorless, although they appear pink or red as the blood within the eye can be seen through the otherwise clear tissue. Albino birds, which have poor vision and depth perception and are highly sensitive to light, are often killed by predators before they can reproduce, so they are extremely rare in the wild.
Leucism, which is perhaps the most common cause of plumage aberrations in birds, is often mistaken for albinism. Leucism is an inherited disorder in which the deposition of melanin is disrupted. Leucistic birds may have completely white plumage or only a few white feathers. Unlike albinos, however, leucistic birds have dark eyes and normal vision. Consequently, they have higher rates of survival and are much more prevalent in wild populations than true albinos.
Perhaps the most well-known cause of odd plumage in waterfowl is hybridization. Crossbreeding is fairly common among many species of birds, and it occurs more often in waterfowl than in any other avian group. In fact, waterfowl may account for more than 60 percent of all known avian hybrids, and over 400 different species combinations have been documented by scientists. Wood ducks alone are known to have hybridized with 26 species, including 16 species of dabbling ducks, five species of diving ducks, and one shelduck.
Several hypotheses may explain why different species of birds hybridize. One cause may be the rarity of a species, as hybridization is more likely to occur when one species is greatly outnumbered by another closely related species sharing the same habitats. However, rarity doesn't fully explain the incidence of hybridization. Mallards and American black ducks, for example, often crossbreed in areas where both species occur in equal numbers.
Hybridization may also occur when nonnative captive waterfowl escape or are released into the wild and subsequently interbreed with closely related native birds. Through a process known as introgression, high rates of hybridization can lead to the decline, and in extreme cases the extinction, of a species. For example, waterfowl managers in Florida are concerned that native mottled ducks could be threatened by hybridization with feral mallards and are now working to reduce the numbers of those aggressive introduced birds.
Exotic waterfowl are birds that are not native to a particular continent. The mute swan is among the most recognizable exotic waterfowl species in North America. Native to Eurasia, mute swans were introduced to the United States in the 1800s, primarily in city parks, large estates, and zoos. There are now breeding populations of feral mute swans in the mid-Atlantic states as well as in the Pacific Northwest and the Great Lakes region. Weighing as much as 30 pounds, mute swans compete with native waterfowl in areas where food supplies are limited. During the past decade, waterfowl researchers estimated that on Chesapeake Bay mute swans were consuming 12 million pounds of submersed aquatic vegetation per year. These submersed aquatic plants, such as wild celery and pondweeds, are vital food sources for redheads, canvasbacks, and other waterfowl. As a result, control efforts were conducted in recent years to reduce the numbers of feral mute swans on Chesapeake Bay and in other areas where they pose a threat to waterfowl habitats.
Other examples of exotic waterfowl species that have been introduced in North America include the Egyptian goose, which is indigenous to Sub-Saharan Africa and southern Egypt, and the mandarin duck, which is native to East Asia. The American Birding Association reports that Egyptian geese began breeding in Southern California during the 1970s and in Florida during the 1980s. The mandarin duck is the Asian ecological equivalent of the wood duck. Although these species are unrelated, they both have spectacular plumage and occupy similar habitats. In North America, mandarin ducks are primarily found in California, where the birds originally became established in Sonoma County. Like wood ducks, mandarin ducks are cavity nesters and will use wood duck nest boxes, but there doesn't appear to be significant competition between these species.
In addition to introduced exotics, several other waterfowl species occasionally turn up in North America on their own. The barnacle goose, a relative of the Canada goose, breeds in Greenland east to Siberia and is an infrequent visitor to this continent. Barnacle geese are most often seen in the Atlantic Flyway, although individual birds have been spotted in other parts of the United States as well. The tufted duck is the Eurasian ecological equivalent of our ring-necked duck. According to sightings posted on birding websites, small numbers of tufted ducks frequent the Pacific and Atlantic Flyways.
However, the most commonly encountered exotic waterfowl species—and the most highly prized by hunters—is the Eurasian wigeon. This close relative of the American wigeon breeds from Iceland, the British Isles, and Scandinavia east to Siberia and Kamchatka. On this continent, Eurasian wigeon are most frequently encountered in the Pacific and Atlantic Flyways. Birds arriving in Alaska and other Pacific Flyway states are likely derived from Siberia, while those in the Atlantic Flyway come from Iceland and northwestern Europe. Like its North American cousin, the Eurasian wigeon feeds both by dabbling in water and grazing on land. It also shares the behavior of stealing vegetation brought up by coots and other waterfowl. In the United States and Canada, Eurasian wigeon are almost always found in the company of American wigeon, and the two species are known to crossbreed, producing hybrid offspring.
Dr. J. Brian Davis is an associate professor of wildlife ecology and management in Mississippi State University's Department of Wildlife, Fisheries, and Aquaculture.
FEATHER FACTS Bird feathers are made of the protein beta-keratin. Protein is, among other things, needed for muscle and feather growth, so waterfowl must consume large quantities of aquatic invertebrates and other protein-rich foods to grow new feathers each year. Two primary pigments determine plumage coloration in birds: melanins and carotenoids. Melanins are found in black, gray, dark-brown, and reddish-brown feathers. Pigmentation derived from melanins is determined genetically and forms during the initial cell divisions of the fertilized egg. Carotenoids are the source of yellow, orange, and red colors in the feathers, legs, and bills of waterfowl. Unlike melanins, carotenoids are acquired by birds from particular foods and transformed into pigments by enzymes.