In an analysis of black duck habitat we determined that the most important factor contributing to whether or not a freshwater wetland staged more than one brood of black ducks, was the stability of the water level. The only explanation I can give for some rather drastic regional decreases in black ducks in the province is extreme flooding during early May, when the majority of black duck females are on the nest. Although flooding is not an abnormal situation in the black duck range, where water levels regularly fluctuate more than a metre, it seldom effects large regions of the range during any year. Therefore, the impact of flooding on waterfowl in the east may be as disastrous as drought in the prairies, except that it occurs most years but on a limited rather than extensive basis. The suggestion that black duck populations are declining in the southern and western portion of their breeding range due to hybridization with and displacement by the mallard, appears valid.
Mallards are simply out-competing and replacing blacks in the area.
Here in the east, at the time when mallard introductions were in vogue, the lack of mallard breeding success was attributed to a susceptibility of the mallard to a blood parasite carried by black flies. In more recent years, the mallard, largely without intentional and organized release programs, has seriously encroached into black duck and black fly range. This eastern movement could be a natural evolution and as uncontrollable as was the eastern movement of the ring-necked duck in the 1940s. Or the influx of mallard might plausibly be correlated with fertility, both natural and man-induced. The mallard seems better equipped to live close to man and his agricultural and domestic activities.