Northern Prairie Wildlife Research Center
Identification of wings as from hybrid ducks is a post facto assessment of the phenomenon that is of greatest interest: the interaction of Black Ducks and Mallards in the wild and the resulting population consequences for each species. Attempts to separate the effects of natural range expansion of the Mallard, anthropogenic habitat change, release of captive-reared Mallards by individuals and governments, behavioral differences between the species, the effects of sport hunting, and hybridization upon population dynamics of Black Ducks and Mallards have been unsuccessful, but not for want of trying. Failure may be attributed to the fact that experiments to detect true differences are difficult to design and conduct, and as a result, studies to date have resulted in universally weak inferences (Nichols 1991). Consequently, the literature remains replete with insufficiently tested, usually single-factor, hypotheses and controversial conjecture (Nudds et al. 1996). Another attempt at a complete listing of research needs for the Black Duck thus would be redundant, but two questions with bearing on the effects of hybridization on the parental populations do seem amenable to investigation.
The first (also raised by Rusch et al. 1989) is, what are the ratios of Black Duck × Mallard pairs to correctly formed pairs in various habitats? The answer to this question would provide an apparent measure of error in mate choice in the wild that could be compared geographically with parts collection survey data and could be compared with changes in species ratios over time in various habitats. In other words, testable hypotheses about the dynamics of hybridization per se could be formed.
The second question is, what are the genetic consequences of hybridization at the rates observed in the wild? Further biochemical studies of Black Ducks and their hybrids, such as those by Patton and Avise (1983, 1985), Kessler and Avise (1984), Oates et al. (1984), Ankney et al. (1986), and Avise et al. (1990), should be devised to elucidate the genetic and population consequences of hybridization to both Black Ducks and Mallards. This effort would initially require laboratory study, but ultimately sampling of wild birds would be needed to elucidate true levels of admixture of gene pools. From this, modeling exercises could be used to address genetic and population consequences of interaction between the two species.
Suggestions for management of Black Ducks have been collected in a series of papers on the species (Spencer 1980, 1981, 1986; Rogers and Patterson 1984; Kirby 1988; Rusch et al. 1989; Conroy and Krementz 1990; and especially criticism of these and similar studies by Anderson et al. 1987; Nichols 1991; and Nudds et al. 1996). Comparison of the calls for action in these later papers with strikingly similar pleas raised 30 years before (Barske 1968) shows that questions regarding management of eastern waterfowl populations have not diminished. Rigorous testing of hypotheses is yet needed to identify cause-and-effect relationships within Black Duck populations. Further studies of hybridization and other population phenomena of Black Ducks and Mallards in areas of sympatry are warranted.