Mallards have been replacing Black Ducks in human-altered landscapes in Ontario, 1996-2019

The American Black Duck (hereafter Black Duck) has received much attention
over the years as a popular game bird species that experienced historical
declines on its wintering grounds. Declines may be due to changes in the
quality and quantity of breeding habitat, competition with Mallards, or
both. Although the Black Duck population has stabilized, spatial variation
in regional population trends of Black Ducks on their breeding grounds can
provide insight into the relative importance of these alternative
hypotheses at provincial and local scales. We analyzed indicated breeding
pairs count data collected in Ontario between 1996 and 2019 through the
Eastern Waterfowl Survey to evaluate the correlation in breeding densities
and trends of the five most abundant waterfowl species. We also tested
whether habitat and anthropogenic disturbance variables could explain the
distribution of those species. Overall, Mallard breeding pair densities
increased across the survey area, while Black Duck breeding pair densities
decreased in the southern and northern part of the survey area but
remained stable in the central part. Densities of breeding pairs of Black
Ducks and Mallards were negatively correlated across survey plots but the
trends in breeding densities were not correlated. Densities of Mallard
breeding pairs increased in human altered landscapes whereas the density
of Black Duck breeding pairs was negatively correlated with agriculture.
Our results suggest that Mallards did not competitively exclude Black
Ducks on the breeding grounds, but instead, Black Ducks were replaced by
Mallards following encroachment of agriculture into Black Duck breeding
habitat. Habitat management plans focused on breeding Black Ducks should
emphasize reduction in the expansion of anthropogenic perturbations and
increases in habitat conservation efforts in the central area of the
breeding range in Ontario.