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Locating, Constructing, and Managing Islands for Nesting Waterfowl

Introduction


Major alterations of the prairie landscape during the last 100 years have been harmful to breeding waterfowl. Large blocks of grassland nesting habitat were converted to cropland, and many feeding and resting areas in wetlands were drained and converted to other uses. Large native predators, which preyed mainly on bison, deer, and livestock, were eliminated and naturally replaced by medium-sized predators that prey extensively on birds and their eggs and young.

Studies of upland-nesting ducks conducted in the last several decades in the northern prairie region revealed nest success rates of only 10 to 20%. Most losses of eggs were attributed to the medium-sized, mammalian predators, including red fox, striped skunk, raccoon, and badger. In addition, many waterfowl hens on nests were killed by red foxes.

To increase waterfowl nest success in this altered landscape, various management tactics were tested by wildlife research agencies. Most of these techniques attempted to make the nest inaccessible to the predator using elevated nest baskets, electric fences, water barriers, or some form of vegetation management.

Islands with few mammalian predators were found to be attractive sites for nesting waterfowl and other bird species because nest success was high. One study of 209 natural islands in the Dakotas and Montana reported high duck-nest densities (3 nests/acre) and high nest success (60%). A study of small rock islands in North Dakota revealed that 19% contained duck nests, mainly mallard, of which 48% hatched. In Canada, 31% of rock islands were used by nesting Canada geese and 6% by ducks. Nest success for Canada geese was 76%.

Island management has been most successfully and widely applied in the prairie pothole region of North America and to a lesser extent in prairies and plains farther west (Figure 1). Lower waterfowl breeding populations and higher numbers of aquatic and avian predators decrease the effectiveness of island management in other regions.

Figure 1: Map of U.S. and Canada -- Primary and Secondary areas of North America most suited to management of nesting islands.
Figure 1.  Island management has been most successfully applied in Prairie Pothole Region of North America.

Common Nesting Species

Most species of waterfowl in the prairie pothole region nest on islands, but mallards and gadwalls usually compose 60 to 80% of the nesting population. Lesser scaup, a diving duck species, is the third most numerous nester. Canada geese commonly nest on islands wherever breeding populations have been established.

Open beaches on islands in the prairie pothole region are favored nesting habitats of shorebirds such as American avocets and piping plovers. Also, colonial nesters including American white pelicans, double-crested cormorants, common terns, and several species of gulls nest primarily or exclusively on islands.

Photo of Gadwall. Photo of American avocet.
Mallard and gadwalls make up 60-80% of the nesting waterfowl populations found on islands. Shorebirds such as American avocets also favor island nesting habitats. (photo by USFWS and Ed Bry)

Management Options

The management of islands that nesting waterfowl use can involve the improvement of natural sites or the construction of new islands. Constructed islands are of two general types. The standard islands are earthen mounds, usually larger than 0.1 acre, built mainly with draglines, bulldozers, and scrapers. Rock islands are constructed by piling rocks in the wetland basin and covering them with soil. Rock islands are small, usually less than 0.01 acre.

Hundreds of natural islands suitable for management are situated in the prairie pothole region. Managing these existing islands is one of the most cost-effective methods to increase waterfowl production. Management of natural islands requires the removal of predators from sites with adequate nesting cover or the removal of predators and planting of nesting cover at sites without cover.

Constructed islands also provide excellent predator-free nest sites, but they are expensive to build. Earthen islands cost about $10,000 to $20,000 per acre. Rock islands cost about $100 each, which is about the same on a surface area basis as earthen islands. Rock islands fill a different role than earthen islands because they are mainly built in small, seasonal wetlands.


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