Many scientists attribute the decline to, among other reasons, increased interbreeding with the mallard, and to the mallard's range expansion into formerly exclusive black duck habitats. Mallards currently comprise 50 percent of Pennsylvania's duck harvest. Habitat Mallard breeding habitat combines shallow-water foraging sites and thick vegetation for nesting. The species prefers open country to woodlands. Ponds, edges of freshwater lakes, sloughs, reservoirs and marshes are ideal.
Mallards often use man-made nesting structures placed over water.
They winter on marshes, lakes, the open waters of rivers and bays. They feed in these places and croplands. Most waterfowl moves away from areas frequented by humans, and consequently have been driven from suitable habitat by expanding towns and cities, rural development and vacation homes. The mallard and the Canada goose, less wary of humans, are occupying much of this altered habitat.