Mallard Use of Nesting Islands Depends on Surrounding Upland Cover

Sustaining duck numbers at or above North American Waterfowl Management Plan population goals will require increased recruitment rates of several species over much of the Prairie Pothole Region. In many parts of the Prairie Pothole Region, only about 1 in 10 mallard nests results in hatched young—predators destroy about 90% of unsuccessful nests.

Waterfowl managers in the prairies employ numerous methods to manage predation on nests and nesting females. One such example is the construction of small nesting islands in large semipermanent and permanent wetlands. Properly constructed and managed islands attract high densities of nesting ducks—especially mallards and gadwalls—and provide safe nesting sites.

Island construction begins with the choice of a location. Some issues related to choosing a location are well understood. For example, wetlands in which islands are constructed must be large enough and deep enough to provide exclusion from mainland mammalian predators, but not so large that wave action causes serious erosion problems. Alkali wetlands are chosen over freshwater ones because the former usually do not support resident mink populations. Islands must be located in areas of high wetland density to ensure adequate numbers of breeding ducks will be present.

Other issues related to choosing a location for an island are not as well understood. Duck production models currently used for planning in the prairies are based on an assumed negative relation between use of islands by nesting ducks and the amount and attractiveness of competing nesting cover in the surrounding landscape. Islands in landscapes dominated by annually tilled cropland theoretically receive higher use than those located in landscapes that are predominantly perennial grass. Prairie Pothole Joint Venture partners from the North Dakota Game and Fish Department, Bureau of Reclamation, Ducks Unlimited, Inc., U.S. Fish and Wildlife Service, and U.S. Geological Survey pooled resources and conducted a 2-year study in North Dakota to test this assumption.

"Approximately four times as many mallard nests were observed on islands in landscapes with 10% grass..."

Effect of surrounding upland cover on use of islands by nesting mallards.

Biologists monitored numbers and success of duck nests on 19 and 22 islands in 1996 and 1997, respectively. Islands were located in landscapes that ranged from about 10% to more than 70% perennial grass cover. Researchers observed 1,746 duck nests of 11 species and nest success of 76% for all species combined. Mallards (35%), gadwalls (30%) and lesser scaup (14%) accounted for most nests.

After considering influences from other variables, use of islands by nesting mallards declined as the area of perennial grass cover in surrounding uplands increased. Approximately four times as many mallard nests were observed on islands in landscapes with 10% grass than on those in landscapes with greater than 50% grass. The study also reaffirmed the importance of dense vegetation, particularly brush, in attracting nesting hens.

The benefit of placing islands in cropland-dominated landscapes is magnified when one considers findings from another Joint Venture study that indicated nest success in uplands was lower in cropland-dominated landscapes than in landscapes dominated by perennial grass cover. Constructed islands can be a cost-effective tool for increasing duck production, if properly located and managed.