By Ryan Boyer and John Coluccy, Ph.D.
The nesting period encompasses only a fraction of the annual cycle of waterfowl, but it is perhaps the most influential time of the year for waterfowl populations. During the nesting period, waterfowl are faced with exhausting physical demands and constant danger while attempting to propagate their species. The number of ducks and geese that ultimately join the fall flight hinges on the ability of nesting birds to overcome a host of challenges and threats.
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.
Once a nest site has been chosen and the nest bowl has been created, the hen will begin laying her eggs, which are collectively known as a clutch. Waterfowl species lay eggs at different rates. Swans lay one egg about every two days, geese lay an egg about every day and a half, and most ducks lay an egg each day until the clutch is complete. Clutch sizes vary by species. The average clutch size is about five for geese and swans, and about nine for diving and dabbling ducks.
The incubation process begins as soon as the first egg is laid. Female waterfowl typically incubate their eggs without any help from males. To line the nest, females add bits of vegetation and pluck feathers from their belly, creating a bare spot known as a brood patch, which allows heat to be more effectively transmitted from their body to the eggs. They also rotate and shift the eggs in the nest to evenly distribute body heat to the entire clutch.
Female waterfowl lose a considerable amount of body mass during incubation. Except for occasional forays into nearby wetlands to preen and replenish energy reserves, the birds spend almost all their time tending the nest. Female ducks take an average of three one-hour nesting breaks each day. Geese and swans generally devote more time to incubation and take fewer nesting breaks than ducks, relying heavily on stored fat to sustain them. The length of the incubation period for waterfowl ranges from 21 to 31 days, and the amount of time devoted to attending the nest increases as incubation progresses.
A variety of factors can influence waterfowl nesting success, including inclement weather. For example, waterfowl nests are sometimes lost to spring flooding and rising water levels. In addition, hail, ice storms, and long periods of heavy rainfall can inhibit the ability of females to regulate the temperature of their eggs during incubation. Early-nesting species such as mallards and pintails are at a greater risk of suffering nest losses to inclement weather than later-nesting species like gadwalls and blue-winged teal.
But avian and terrestrial predators are often the primary cause of waterfowl nest losses. Mammals such as skunks, opossums, mink, long-tailed weasels, ground squirrels, red foxes, and domestic dogs and cats are the most common terrestrial nest predators, although snakes also occasionally eat waterfowl eggs. Owls, hawks, crows, ravens, seagulls, and magpies are among the most common avian predators of waterfowl nests.
Of all the challenges faced by nesting waterfowl, however, the most serious is habitat loss. The United States has already lost more than half its original wetlands and 70 percent of its grasslands, and these vital habitats continue to be lost at an alarming rate. Ducks Unlimited works with a variety of partners to conserve vital waterfowl breeding habitat in the Prairie Pothole Region, Western Boreal Forest, and other high-priority areas. Public policies such as Farm Bill conservation programs that provide incentives for landowners to restore and enhance wetlands and grasslands on less productive agricultural lands are vital to meeting DU's conservation objectives. In addition, DU works with farmers and ranchers to protect intact waterfowl habitats with conservation easements and to incorporate winter wheat into their crop rotations to provide more secure nesting cover for pintails and other upland-nesting ducks. By supporting these and other DU conservation efforts, you are helping to ensure that when waterfowl return to the breeding grounds in the spring, the birds will find the habitat they need to successfully nest and raise their broods.
Ryan Boyer is a master of science candidate at Michigan State University. Dr. John Coluccy is director of conservation planning in DU's Great Lakes/Atlantic Region.