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Locating, Constructing, and Managing Islands for Nesting Waterfowl

Location Factors for Earthen Islands

The location of natural islands or new construction sites can be obtained by querying local and regional resource managers or by examining existing aerial photography or National Wetlands Inventory data. Aerial photographs can be examined at local U.S. Department of Agriculture, Agriculture Stabilization and Conservation Service offices or purchased from USDA-ASCS, P.O. Box 30010, Salt Lake City, UT 84130-0010. National Wetlands Inventory maps can be obtained from the U.S. Geological Survey, Box 25046, Stop 504, DFC, Denver, CO 80225.

Aerial photographs of Alberta, Canada can be obtained at Maps Alberta, Land Information Services Division, 2nd Floor, North Petroleum Plaza, 9945 - 108 Street, Edmonton, AB, Canada T5K 2G6. Aerial photographs of Manitoba and Saskatchewan are ordered from the Surveys and Mapping Branch, 1007 Century Street, Winnipeg, MB, Canada R3H 0W4 and at the Map and Air Photo Distribution Center, 1st Floor, 2045 Broad Street, Regina, SK Canada S4P 3V7.

To maximize cost effectiveness, the location of natural islands that are to be managed or the construction of earthen islands should be selected carefully. Biological concerns must be given primary consideration in the planning process, although strongly weighted by economic and engineering factors. For instance, earthen islands should not be managed or constructed close to shore or in shallow, heavily vegetated wetlands even though the cost may be low. At poor sites, few birds will be raised and the price per fledged bird will be high. Also, earthen islands should not be built at good sites far from shore in open, alkaline wetlands where construction is expensive because the cost of raising a bird will again be high.

Distance to Shore and Emergent Vegetation

The most successful nesting islands are farther from shore (preferably 400 to 1,500 feet) where they are less accessible to mammalian predators. In large wetlands, however, duckling mortality may be increased on islands that are too far (>1,500 feet) from the shore. Ducklings hatched on islands that are far from shore must cross long stretches of open water, where they may be lost to gulls, other predators, or violent wave action.

Islands that are placed away from emergent vegetation are safer nesting habitat than islands near emergent vegetation. Emergent vegetation provides cover and access routes for mammalian predators. Also, nesting waterfowl pairs on islands prefer open, grassy shorelines and avoid shorelines shielded with tall emergent vegetation or shrubs.

Wetland Type

The most productive nesting islands are usually in large, alkaline wetlands. Alkaline wetlands provide limited habitat for mammalian predators because there is little food in the form of small vertebrates and little emergent plant cover. Ironically, the bleak qualities of alkaline wetlands create a positive benefit for waterfowl in the form of highly successful nesting islands. Alkaline wetlands are those labeled brackish, subsaline, or saline by Stewart and Kantrud (1971) or mesosaline, polysaline, eusaline, or hypersaline by Cowardin et al. (1979). The characteristic emergent plant in alkaline wetlands is alkali bulrush, but often these wetlands contain few emergent plants. The primary open-water plants are widgeongrass, muskgrass, and sago pondweed.

Islands in freshwater wetlands can provide good waterfowl nesting sites provided there is an adequate open-water barrier. Open water with sufficient depth decreases the probability of predators walking or swimming to the island from the mainland. However, the cost of islands becomes more expensive as water depth increases because additional earth fill is required. It is probably too costly to place islands where the normal water depth exceeds 3 or 4 feet.

Wetland Complexes

To be successful, nesting islands must be within 1 mile of a complex of wetlands (preferably ≥40 basins or acres) to support waterfowl breeding pairs and broods. Mallards and gadwalls are usually important nesting species on islands because they can move to them from wetlands that are 1 to 2 miles away. Species such as blue-winged teal and northern shovelers are usually not common nesting species on islands as they will only move 1,000 to 2,000 feet to them from nearby wetlands that provide food and cover. Wetland complexes should contain both seasonal and semipermanent ponds to provide important food and cover for nesting hens and broods. Wetlands are particularly important adjacent to alkaline or deep freshwater wetlands, which often lack adequate food and cover for waterfowl.

Aerial photo of a wetland complex.
To be successful, nesting islands must be close to good wetland complexes that support waterfowl breeding pairs and broods. A good wetland complex would typically include 40 or more basins within 1 mile of the island. (photo by Northern Prairie Wildlife Research Center)

Size and Spacing

There is no known association between size (acres) and spacing (distance apart) of islands and use by nesting waterfowl. Consequently, we treated the following size and spacing recommendations from the perspective of economics and waterfowl breeding behavior.

Waterfowl species that commonly nest on islands are able to move long distances from wetland feeding and resting areas to islands. These species are also able to concentrate nests into small areas. As a result, constructed nesting islands can be small and widely spaced. The management of existing islands must be considered quite differently because there is no cost of establishment. Therefore it is cost-effective to manage existing islands as small as 0.1 acre and to manage many sites at a single location.

Generally, an investment of not more than 1 acre of constructed islands in a square mile of prairie-pothole habitat is recommended. Where sufficient wetland habitat is available, it may be preferable to build 2 or 3 islands, totaling 1 acre within the square mile rather than a single island. This practice increases costs but reduces the risk of total nest loss at one location. Creating 2 or 3 islands in a square mile makes the islands accessible to more breeding duck pairs, resulting in more nests. More islands also reduces territorial strife between nesting pairs of Canada geese, probably resulting in an increased number of nests. When 2 or more islands are built within 1 square mile, islands should be separated by at least 300 feet.

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