Behavioral and Ecological Factors Contributing to American Black Duck-Mallard Hybridization

Abstract

Investigation of courtship and pair formation of a wintering population of American black ducks (Anas rubripes) and mallards (A. platyrhynchos) near Ottawa, Ontario, indicated that initially drakes of both species exclusively courted and paired intraspecifically. After all female mallards had paired, the remaining mallard drakes joined black duck courtship groups. Of the 33 unpaired black duck females remaining at this time, only 27% formed intraspecific pairs, whereas 73% selected mallard drakes as mates, despite there being an excess of black duck drakes. Based on these results, a scenario for black duck-mallard hybridization is proposed. It involves ecological factors including the male-biased sex ratio in northern wintering populations, artificial feeding, and roost-site limitation. Other behavioral aspects, such as the earlier pair formation in mallards and the superiority exhibited by mallard drakes when competing for black duck females are discussed. J. WILDL. MANAGE. 48(3):846-852 One factor essential to a species' genetic integrity is its continued reproductive isolation from congenerics through geographic, habitat, or behavioral means (Lack 1971). Two species of dabbling duck, the American black duck and the mallard, are similar both morphologically and behaviorally, and their hybrids appear to be as viable and as fertile as the parental species (Johnsgard 1961). However, geographic range and evolved habitat differences have until recently deterred hybridization between these two species. Prior to 1900, the black duck and mallard were geographically isolated. Heusmann (1974) examined the evolutionary history of the black duck and hypothesized that it evolved during the Pleistocene glacial period when a population along the eastern coast of North America became isolated from western mallard populations. Habitat differences further segregated these populations as the black duck began breeding in the forested regions of the east and the mallard nested in grassland habitats of the west. In the present century, however, land use changes, including the deforestation of the eastern region, have caused a decline in the available breeding habitat of black ducks while simultaneously providing favorable habitat for nesting mallards. Johnsgard (1967) concluded that these environmental modifications were facilitating the colonization, by mallards, of extensive eastern areas formerly occupied by the black duck. The invasion of the mallard into black duck breeding territory has also resulted in the convergence of these two species on wintering grounds. Since the majority of pair bonding in both species occurs throughout the winter (Stotts and Davis 1960, Lebret 1961) and the courtship displays that lead to pair formation are similar (Johnsgard 1960), interspecific pairing can readily occur where these two species winter together. As a result of black duck hybridization with the much larger mallard gene pool and the constant reduction of its nesting habitat, it is believed by many that the black duck as a pure species will eventually disappear. The objectives of this study were to examine courtship activity and pair formation in 846 J. Wildl. Manage. 48(3):1984 This content downloaded from 157.55.39.110 on Sat, 24 Sep 2016 06:00:00 UTC All use subject to http://about.jstor.org/terms BLACK DUCK-MALLARD HYBRIDIZATION * Brodsky and Weatherhead 847 black ducks and mallards wintering together at the northern limit of their range and to determine the factors contributing to interspecific pairing between these two species. We thank H. G. Merriam for suggesting the suitability of the Manotick waterfowl population for this study, C. A. Barlow for comments on an earlier draft of this manuscript, and J. Sharkey-Thomas for feeding the ducks. Financial support was provided by the Nat. Sci. and Eng. Counc. of Can. and Carleton Univ.