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[In comparison with the typical Mallard wing]...good procedures for this type of identification are lacking. Birds intermediate in color between the two species are considered hybrids. These normally have a poorly defined white stripe anterior to the speculum ... . Although we only look on the underwing of birds that are suspect, the presence of a few dark brown feathers usually causes us to consider the bird a hybrid. Birds considered to be "hand-reared" are those with extra white on the tertials, dark underwings, or odd proportions of length to width (a few of the latter are unusually large wings). Unfortunately, the major area for both crosses and "hand-reared" birds appears to be the same, i.e., southern New England to Virginia.

Major difficulties in identifying wild Black Duck × Mallard hybrids arise from (1) the lack of obvious sexual dichromatism in the Black Duck; (2) the existence of a number of plumage characteristics, often referred to as inherent "Mallard" characteristics, in typical Black Duck plumage (Phillips 1912, 1915, 1921); (3) substantial plumage variation among individuals possibly related to nutrition and stress (cf. Kirby and Fredrickson 1990); (4) a rather broad range of "typical" plumage features in both species; (5) considerable geographic variation in plumage of both species; and (6) interbreeding of wild Black Ducks with released or otherwise feral Mallards throughout much of historic Black Duck breeding and wintering range. Further, uncertainty about what constitutes "typical" Black Duck plumage has stymied attempts to determine hybridization incidence (e.g., Heusmann 1974; Johnsgard and DiSilvestro 1976; Morgan et al. 1978, 1984b), even though most investigators agree that plumage coloration per se (Bellrose 1976) or plumage coloration plus the "curly" tail feather character of male Mallards (Livezey 1991) can be used to distinguish the two species even in the absence of other useful morphometric variables (cf. Hanson and Ankney 1994). The range of variation in Black Duck plumage has not been described and only limited descriptions of Black Duck × Mallard hybrid plumage are available (Dutcher 1889; Eaton 1903; Bigelow 1907; Phillips 1915, 1921; Kortright 1942). Palmer (1976:328) summarized what little is known of this variation:

...the Black has considerable individual variation in characters of feathering, some geographical variations in these, some clinal variations in size (larger northward), and the effects of limited crossing with Mallard and perhaps Mottled Duck are somewhat obscured by imprecise knowledge of the full extent of variation in "pure" Black Ducks.