Northern Prairie Wildlife Research Center
The most widely used post structures for mallards are open-topped cones (often called "baskets") and horizontal cylinders (Figure 7). Open-topped cones can also be used by Canada geese, which may prevent mallard use. If open-topped cones are targeted for mallards, the top diameter should be no greater than 20 inches and a wreath of grass should be wired around the rim to provide lateral cover. The remainder of the cone, up to the wreath, can then be filled with grass nest material (see "Nest Material and Maintenance" section). When commercially available fiberglass cones are used, a 6-inch wide hardware cloth (1/4 to 1/2-inch mesh) ladder should be attached to the side of the cone to help ducklings exit. Holes must be drilled in the bottom of any solid cone to provide drainage.
Cylinders provide the lateral and overhead cover mallards prefer, retain nest material better than open-topped cones, help protect the hens from avian predators, and exclude Canada geese. Although not extensively tested in Iowa, cylinder use as high as 80 percent has been observed where open-topped cone use was only 40 percent. The cylinder structure most widely used consists of a 2 to 3-foot long 12-inch diameter cylinder inside a 14-inch diameter welded wire cylinder (Figure 7). A landing platform at one end of the cylinder, at least 10 inches long and 8 inches wide, makes hen access easier. Grass is placed between the inner and outer cylinders to provide lateral and overhead cover and must be refilled annually (see "Nest Material and Maintenance" section). To make the annual task of putting grass between the inner and outer cylinders easier, the outer cylinder should be constructed so that it can either be opened along its length at the top or at the bottom where it attaches to the 2x4. Approximately 6 to 8 inches of grass hay must also be laid in the bottom of the inner cylinder to be used as nest material.
Mallard post structures should be installed using the guidelines and techniques described in "Canada Geese - Post Structures". However, in contrast to geese, mallards have shown reluctance to use high, exposed, nest structures. A good rule-of-thumb is to keep the rim of the structure just above the height of the old cattails or bulrushes or 3 feet above the high water line. Structures placed further than 50 feet from shore are less likely to be visited by predators. In marshes, the preferred locations are at the edges of emergent vegetation (cattails, bulrushes) or in small pockets of open water. Structures located in dense vegetation are seldom used.