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Description and Identification of American
Black Duck, Mallard, and Hybrid Wing Plumage

Introduction


The American Black Duck (Anas rubripes) (hereafter Black Duck) is a priority concern of waterfowl managers as described in the North American Waterfowl Management Plan (Canadian Wildlife Service and U.S. Fish and Wildlife Service 1986) because its populations have declined. This decline has been attributed to loss and degradation of habitat, harvest by hunters, and competition and hybridization with Mallards (A. platyrhynchos), which are expanding their range eastward (Rogers and Patterson 1984; Ankney et al. 1987, 1988; Kirby 1988; Rusch et al. 1989). Relative numbers of the two species in many portions of the East have changed (Collins 1974; Dennis et al. 1984; Ankney et al. 1987; D'Eon et al. 1995). Continued release of captive-reared Mallards and the establishment of resident, feral Mallard flocks have provided opportunity for increased interspecific matings between Black Ducks and Mallards. Some waterfowl ecologists believe that introgressive hybridization has been and will continue to have an increasingly detrimental influence on Black Ducks (Heusmann 1974, 1988; Spencer 1980, 1986; Ankney et al. 1987; Kirby 1988), but biologists differ on the consequences of hybridization for the Black Duck gene pool (Ankney et al. 1986, 1987, 1989; Ankney and Dennis 1988; Hepp et al. 1988; Conroy et al. 1989). Because of these disagreements, population monitoring schemes need estimates of year-specific incidence of hybridization and the long-term trend in hybridization.

Early attempts to assess the incidence and trend of hybridization between the two species with electrophoretic techniques failed except in identifying first filial generation (F1) hybrids and the parental generation (P1) (Morgan et al. 1976, 1978, 1984a, 1984b). Subsequent biochemical investigations (Patton and Avise 1983, 1985; Oates et al. 1984; Ankney et al. 1986; Avise et al. 1990) and phylogenetic (cladistic) analyses (Livezey 1991 and review therein) confirmed that Black Ducks and Mallards are closely related but did not provide information useful in identifying hybrids.

A true estimate of hybridization rate, that is, the proportion of mixed pair bonds formed in the wild or the actual numbers of hybrids hatching from mixed-species matings, has been obtained only in limited, accessible areas (e.g., Longcore et al. 1987; Sanderson 1993; D'Eon et al. 1994). No schemes to obtain such data on a large scale in the wild have been attempted. Instead, measures of interaction between Black Ducks and Mallards have been confined to documenting shifts in the breeding range (e.g., Collins 1974; Dennis and North 1984a, 1984b; Dennis et al. 1984, 1989; Ross and Fillman 1990), examining changes in the relative abundance of the two species in annual Mid-winter Waterfowl Survey counts (Steiner 1984) or Christmas Bird Counts (Wing 1943; Johnsgard 1961; Johnsgard and DiSilvestro 1976), and calculating the fraction of the wings submitted to the annual U.S. Fish and Wildlife Service and Canadian Wildlife Service Parts Collection Surveys putatively identified as hybrids (Johnsgard 1967; summaries in Carney et al. 1975, 1983). Only this last measure attempts to address frequency of hybridization per se in wild populations.

Despite the limitations of the parts collection surveys (Martin and Carney 1977; Boyd and Finney 1978), they are valuable information tools for management (Boyd et al. 1974). It is the only technique currently capable of detecting changes in Black Duck × Mallard hybridization over the entire range of sympatry. In the early 1970's, the Canadian Wildlife Service (Québec Region, Ste-Foy, Québec) and two U.S. Fish and Wildlife Service (now U.S. Geological Survey) entities (Northern Prairie Wildlife Research Center, Jamestown, North Dakota, and Patuxent Wildlife Research Center, Laurel, Maryland) independently began studies to refine means of detecting Black Duck × Mallard hybrids. These efforts were combined in 1976. Between 1976 and 1983, field, laboratory, and museum studies were conducted. Since 1984, the techniques established were further evaluated in the field; oral presentations on the techniques also resulted in important feedback. In this paper, we present the sum of more than two decades of evaluation of the issue, provide an improved key for identification of Black Duck × Mallard hybrids, and reinterpret previously published data on Black Duck × Mallard hybridization.


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