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Locating, Constructing, and Managing Islands for Nesting Waterfowl

Rock Islands


Locations Factors

Unlike earthen islands, rock islands are usually placed in seasonal wetlands, often close to shore, within stands of emergent vegetation. Rock islands function like large nest structures. Rock islands are probably successful because they are partially safeguarded from upland predators by water barriers yet they are too small to attract aquatic predators such as mink.

As a general rule, no more than 1 rock island should be placed in each 20 acres of wetland habitat. No more than 20 should be built in 1 square mile of prairie-pothole habitat. Space rock islands at least 100 feet apart.

Construction Guidelines

Rock islands should be built when wetlands are sufficiently dry to support heavy equipment. Construction in winter is usually not necessary because seasonal wetlands, where rock islands are usually built, normally are completely dry by late summer.

Rock islands are built primarily of rocks piled in a wetland basin to a height of 2 to 3 feet above the average water level. Another 2 to 3 feet of soil from the marsh bottom or from adjacent upland sites is placed on top of the rocks. The completed islands are only 10 to 15 feet in diameter.

Management

On rock islands, nesting cover will usually develop from seeds contained within the soil covering. However, it is probably worthwhile to incorporate 3 or 4 ounces of seed into the top 1 inch of soil as a means of accelerating the vegetative process. Seeds of such plants as sweetclover, tall wheatgrass, and intermediate wheatgrass would be appropriate.

Because rock islands are small and widely dispersed it is not cost-effective to visit each one annually to trap predators. However, a representative portion of rock islands should be occasionally visited to determined use and success by nesting waterfowl.

Photo of man standing in wetland next to a rock island.
Rock islands, consisting of rocks piled in a wetland basin to a height of 2 to 3 feet above the average water level and covered with 2 to 3 feet of topsoil offer safe, secure nesting sites. (photo by South Dakota Cooperative Wildlife Research Unit)


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