Northern Prairie Wildlife Research Center
Locating, Constructing, and Managing Islands for Nesting Waterfowl
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.
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.
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.
|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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