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Locating, Constructing, and Managing Islands for Nesting Waterfowl

Management of Earthen Islands

Successful waterfowl nesting islands must contain a combination of proper nesting cover and few nest predators. Islands contain adequate duck nesting cover when 50 to 75% of the area is covered by tall (≥1.5 feet) grasses, forbs, legumes, and weeds or low shrubs (≤4 feet).

Canada geese prefer to nest in short vegetation that permits good visibility of the surrounding landscape. Overall, Canada geese are quite adaptable in their nesting habitats but they generally avoid low shrubs, tall shrubs, and trees.

Photo of Mallard nest containing eggs surrounded by western snowberry.
A Mallard nest located in a stand of western snowberry. Western snowberry is a preferred nesting cover of both mallards and gadwalls. (photo by Northern Prairie Wildlife Research Center)

Seeding Grass-Legume Nesting Cover

Grass-legume mixtures should be seeded on an exposed seedbed free of competing plants (Duebbert et al. 1981). Bare, weed-free seedbeds are available on new islands. However, on existing islands where vegetation is being replaced, standing plants must be controlled and the ground cleared of vegetation prior to seeding. Competing plants can be suppressed by repeated cultivations or with wide-spectrum herbicides. Removal of standing plants by burning or with herbicides followed by burning has worked to prepare seedbeds in some situations.

Grass seedings should be completed soon after the island is constructed. Seeding in winter as the last phase of construction has worked well because heavy equipment is available at the site and a good stand of nesting cover is usually established. Early spring and late fall are also good times to seed nesting cover. Small-grain drills or no-till drills should be used for seeding. A mixture of tall wheatgrass, intermediate wheatgrass, alfalfa, and sweetclover provides good grass-legume nesting cover (Figure 3). Most common grass and legume seeds can be obtained locally at seed dealers or grain elevators (Appendix A).

Grass-legume seedings should be visited at the end of the second growing season to evaluate stand establishment. Stands where 60-70% of the plants survived would be considered successful. Stands where fewer than 60% of the plants survived would be considered unsuccessful and managers would probably have to replant or reseed.

The vigor and attractiveness of grass-legume cover to nesting ducks tends to decline over time. Plant vigor can sometimes be restored by cultivating the soil to a depth of 4 to 6 inches during early spring or late summer. However, in many situations plant vigor can be restored only by destroying the existing vegetation and reseeding.

Tall wheatgrass 11.0 × 0.40 ×   =  
Intermediate wheatgrass 10.0 × 0.38 ×   =  
Alfalfa 4.4 × 0.22 ×   =  
Sweetclover 3.4 × 0.15 ×   =  
a Seed quality is determined by dividing 10,000 by PG. PG is determined by multiplying percent seed purity × percent seed germination.
Figure 3.  A mixture of tall and intermediate wheatgrass, alfalfa, and sweetclover seeded according to the rates specified in this table will provide a dense stand of nesting cover.

Planting Low Shrubs

Western snowberry and Wood's rose are 2 low shrub species preferred as nesting cover by mallards and gadwalls. Both shrub species are available from commercial nurseries (Appendix B). Low shrubs normally require hand labor to plant and maintain weed-free but, once established, provide attractive nesting cover for many years (Figure 3).

Low shrubs should be planted in small plots (0.2 to 0.4 acre in size) and into fertile topsoil where weed competition has been eliminated. The shrubs should be planted at a 2.5-foot spacing. Planting should be done in mid-May to mid-June after the threat of frost has passed. Weeds should be removed during the first two growing seasons to reduce competition for space, moisture, and nutrients. This can be accomplished by 2 or 3 tillage passes or by hand hoeing. During the first fall, after warm temperatures have passed, a chemical such as dichlobenil can be applied to control weeds during the second growing season.

To eliminate the need for physical or chemical weed removal, shrubs can be planted into a plastic weed-barrier mulch (14 mil polypropylene). The plastic mulch is laid down before planting and the sheet is cut where each plant is placed. Plastic mulch can be put into place by machine when it is possible to drive to the island, but hand labor will be required on islands surrounded by water. The plastic mulch is anchored automatically when applied by machine but is held down by rocks and soil when it is applied by hand. Shrub survival can be improved on all types of planting systems by watering plants during the growing season. Water readily moves through the plastic mulch. Shrubs should be established by the end of the second growing season.

Photo of newly planted plot of shrubs.
This photograph shows a newly planted stand of western snowberry and Wood's rose. Low shrubs should be planted in small plots at a 2.5 foot spacing. (photo by Northern Prairie Wildlife Research Center)

Photo of planted plot one year later.
This is the plot that was shown in the above photograph, only one year later. Weed removal accomplished through mechanical or chemical means is required for shrub establishment. (photo by Northern Prairie Wildlife Research Center)

Removing Tall Shrubs and Trees

Waterfowl generally avoid nesting under tall shrubs (>4 feet), such as northern hawthorn and wild plum, and trees. The foliage from tall shrubs and trees shades out low ground cover and also provides perching and nesting sites for avian predators. Tall shrubs and trees should be removed on nesting islands by cutting and spraying regrowth with an approved broadleaf herbicide.

Branches, wood debris, or dead litter that remain after the shrubs and trees are cut down can be cleared by burning. However, the fire should not be allowed to spread to the remainder of the island. Burning generally is not a good management tool for nesting islands because fire can eliminate desirable duck nesting habitats.

Livestock and Fencing

Duck nesting cover that has been reduced on islands by grazing can often be restored by excluding livestock with fences. Frequently, much of the plant removal occurs in fall, when cattle reach islands by crossing dry or shallow wetlands. To halt fall grazing, additional fencing may be required or an agreement might be made with the livestock owner to change the grazing period.

Managing Predators

Managed islands must be relatively free of mammalian predators to provide successful waterfowl nest sites. Normally, small islands (≤3.5 acres) have few resident or visiting predators and annual trapping is rarely required. Small islands might be visited every few years to assess nesting use and success. Most large islands (>3.5 acres) must be trapped annually. Trapping can be accomplished using quick-kill body traps set in baited boxes (Figure 4) or, if necessary, leg-hold traps. Predator trapping should extend from early April, when the wetlands become ice-free, until mid-July, when most nesting is completed.

Trapping should be restricted to the islands. Traps should be dispersed in the uplands and along the shorelines to capture different predators using different habitats. The most productive trapping sites usually are along shorelines and near rock piles, debris, or dens. Trapping can be facilitated by placing 5- to 10-foot lengths of 8- to 12-inch diameter culverts along the shoreline. The center of the culvert could be buried but the ends should remain open to create attractive locations to capture mink, raccoon, and other small carnivores.

Avian predators, other than gulls, usually are not a primary concern on islands in the prairie pothole region and regions to the west. However, in certain situations, particularly where there are trees nearby, American crows or magpies may destroy duck eggs and raptors may kill nesting hens. To reduce the threat of these avian predators, tall shrub and tree perching sites should be removed from islands.

Sometimes, ducks nest on islands where ring-billed gulls and California gulls are numerous. In these situations any advantages gained by increased nest success can be offset when gulls prey upon the ducklings produced. Both species of gulls nest on islands, where they use bare ground as loafing and nesting sites. It may be possible to deter gull presence by establishing low shrub or tall grass and forb cover on the entire island surface.

If it is not possible to establish sufficient cover to discourage nesting gulls they might be deterred using lines and wires. Wire and monofilament fishing line have been used to discourage feeding gulls, and yellow mason line has been used to deter breeding gulls. Position the lines over the nesting area in parallel rows set about 6 feet apart and 3 feet above the ground. The lines can be attached to steel bars or wooden posts pounded into the ground.

Photo of skunk captured using a quick-kill body trap. Figure 4: Diagram of a quick-kill body trap box.
Figure 4.  Quick-kill body trap set in a wooden box. Quick-kill body traps set in boxes baited with sardines and other attractors can be effective in controlling potential nest predators such as the striped skunk. (photo by Northern Prairie Wildlife Research Center)

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