Northern Prairie Wildlife Research Center
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.
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.
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.
|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)|
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.