HABITAT USE BY MALLARDS AND AMERICAN BLACK DUCKS BREEDING IN CENTRAL ONTARIO

Mallards (Anas platyrhynchos) were virtually absent from central Ontar io as recently as 1948 but now exceed 60 pairs/100 km2 in some areas. Black Duck (A. rubripes) numbers in central Ontario appear stable with breeding densities in some areas exceeding 40 pairs/100 km2. Densitie s of breeding Mallards and Black Ducks exhibit spatial variability sug gesting that habitat quality may influence distributions. We used data from Canadian Wildlife Service breeding pair surveys to classify wetl ands as used only by breeding Mallards, used only by breeding Black Du cks, shared by Mallards and Black Ducks, or vacant (i.e., not used by either species). We evaluated wetland fertility by analyzing water che mistry (color, pH, conductivity, alkalinity [CaCO3], calcium, magnesiu m, sodium, potassium, total phosphorus) and physical characteristics ( shoreline irregularity index [SI], percent open water, wetland size) f or 447 wetlands. Wetlands that Mallards shared with Black Ducks or sol ely occupied were the most fertile. Vacant wetlands were least fertile . Areas that supported few Mallards or Black Ducks had relatively infe rtile wetlands; Mallards predominated in areas with relatively fertile wetlands. We conclude that wetland fertility has influenced the distr ibution of Mallards and Black Ducks in central Ontario. Mallards and B lack Ducks apparently prefer wetlands with similar characteristics (i. e., high fertility, moderate open water, high SI, and small size), so competition for breeding sites is likely. Historical data from Ontario lead us to suspect that this competition may have contributed to the decline in Black Duck numbers.