American black ducks nest throughout the Chesapeake Bay watershed, from the rivers and streams of Pennsylvania to the tidal freshwater wetlands of Virginia. Thousands of black ducks also migrate from breeding grounds in the extreme northeastern United States and southeastern Canada to overwinter in Bay tributaries and wetlands.
Although most of the Bay’s migratory waterfowl are increasing, the number of both migratory and resident black ducks continues to decline. Once the premier dabbling duck pursued by hunters, their numbers have dropped from more than 100,000 in the mid-1950s to around 30,000 in the late 1990s.
According to Doug Forsell, an ornithologist with the U.S. Fish & Wildlife Service’s Chesapeake Bay Field Office and chair of the Bay Program’s Waterfowl and Other Waterbirds Workgroup, the largest declines have been in Maryland’s Upper Bay. The decline in both breeding and migrating populations can be attributed to habitat loss — on the Bay and in the North. But many experts also blame competition for prime breeding habitat from the rising resident mallard population and overhunting.
Black ducks use wetlands for nesting and raising young. In fact, black ducks need two types of habitat to successfully raise young, They need a secure nesting area, with cover from overhead predators, like gulls, and isolation from land-based mammal predators, like foxes, rats and raccoons. Soon after hatching, parents move young ducklings from the nest to “brood habitat,” usually in marshes teeming with underwater Bay grasses, insect larvae and other invertebrates. These two habitat types must be close enough together for young ducks to survive the swim.
In the past, islands provided excellent habitat for breeding black ducks. Their isolation from land-based predators and habitat gradients from uplands with trees and shrubs to surrounding marshes with emergent and submerged plants were perfect. But islands have been rapidly disappearing from Chesapeake Bay, the result of erosion and sea level rise.
Black ducks also nest in forested and emergent wetlands, but those habitats are also disappearing. Development that fills wetlands and shores up the edge with bulkheading permanently removes crucial nesting and brood habitat for these shy ducks. The best brood habitat used to be the Upper Chesapeake’s estuarine bays, which were once choked with underwater Bay grasses. Patuxent Wildlife Research Center biologist Mike Haramis says that the loss of both these grass meadows and the diversity of invertebrates that lived there has been the single most devastating loss of habitat for resident, breeding American black ducks.
Competition for breeding habitat between black ducks and mallards is a subtle phenomenon. Both birds can easily interbreed and their habitat requirements are similar. Originally, mallards were a prairie-nesting species that did not breed in the East, but did overwinter on the Bay. Surveys showed that as recently as the late 1950s, mallards were still a rare breeder in the East. Today’s Chesapeake Bay breeding populations were probably bolstered by both state-supported and private releases of hand-raised mallards for hunting.
Black ducks shy away from humans, whereas mallards are a bit bolder and more willing to nest and raise young near human structures and activities. Haramis notes that, “black ducks are ecologically incompatible with humans, but mallards are a more adaptable and cosmopolitan species.” In addition, the breeding of mallards by humans may have skewed the Bay’s stock toward a more human-acclimated bird. As prime breeding habitat disappears, mallards use marginal habitats near humans, while black ducks have neither adapted to human disturbance nor landscapes.
The “North American Waterfowl Management Plan” initiated an international “Black Duck Joint Venture” in 1986, which supports long-term research and a banding program. Banding data provides biologists and managers with information on black duck populations and harvest effects. Field research explores habitat use, competition with mallards and habitat needs. This data is just beginning to help managers regulate harvests and understand mallard/black duck interactions.