The habitats used by waterfowl for nesting vary greatly by species and often among individuals of the same species. Based on their nesting habitat preferences, ducks are grouped into three general categories: upland-nesting species, overwater-nesting species, and cavity-nesting species. Upland-nesting ducks include most of the dabbling ducks such as blue-winged teal, mallards, northern pintails, gadwalls, and American wigeon. Overwater nesters consist primarily of diving ducks like redheads, canvasbacks, greater scaup, and ring-necked ducks. And cavity-nesting waterfowl comprise species such as wood ducks, buffleheads, common goldeneyes, and hooded mergansers. In some cases, certain species will nest in more than one habitat type and thus cannot be easily placed in a single category. Mallards, for example, are generally considered to be an upland-nesting species, but are known to nest in a variety of locations, including in overwater vegetation, trees, artificial nesting structures, and even the occasional backyard flower pot.
After a hen selects a nesting site, her next task is to create what is known as a nest bowl. Upland nesters often make a shallow depression in the ground called a scrape. Cavity-nesting species make their nests in recessed locations, usually in holes in dead and decaying trees as well as in artificial nest boxes. Overwater nesters typically build their nests in flooded cattails, bulrushes, or willows and on floating mats of woven vegetation.