Understanding Waterfowl: The Amazing Molt

Waterfowl replace all of their feathers with new plumage at least once a year

© Michael Furtman

By Mark Baschuk and Stuart Slattery, Ph.D.

Feathers are vital to waterfowl in many ways beyond their essential role in flight. They provide insulation, flotation, and camouflage, and drakes use their brightly colored breeding plumage to attract mates. While feathers are remarkably durable, they eventually wear out over time.

Waterfowl replace their old plumage with new feathers at least once a year during a process known as molting. Whistling-ducks, geese, and swans undergo a single annual molt, replacing all body, wing, and tail feathers shortly after the nesting season. Most ducks, however, undergo two molts a year. The first molt occurs shortly after nesting. Drakes trade their gaudy breeding plumage for drab brown feathers known as "basic" or "eclipse" plumage. The second molt occurs from fall to early winter. Only the birds' body feathers are replaced during this molt, in which drakes develop their brightly colored "alternate" or "nuptial" plumage. Of course, there are exceptions to these molting patterns in ducks. For example, drake ruddy ducks develop their alternate plumage in spring, while drake long-tailed ducks molt their body feathers three times a year.

The manner in which waterfowl molt their flight feathers, or primaries, is unique among birds. Most birds undergo a "sequential molt," in which their flight feathers are lost one at a time from the innermost primary feather to the tip of the wing. This allows many birds to retain their flight capabilities while molting. Waterfowl undergo a "simultaneous wing molt," losing all of their primary feathers at once, which renders them flightless for 20 to 40 days. Waterfowl are well adapted to survive during this flightless period because they inhabit wetlands, which provide food, shelter, and safety without the immediate need to fly. Molting geese continue to graze on land while flightless, but they never travel too far from water and are excellent runners.

Although wetlands provide the basic necessities for survival, flightless waterfowl are still vulnerable. As a result, ducks and geese have developed several physical and behavioral adaptations that increase their chances of survival. The drab coloration of eclipse plumage is one such adaptation, allowing drakes to escape detection more easily than they would in their bright nuptial plumage.

Molting waterfowl typically select wetlands that best suit their escape strategies. For example, dabbling ducks such as mallards, northern pintails, and blue-winged teal typically hide in dense vegetation when threatened, so these ducks gather to molt on large permanent wetlands with dense stands of bulrush or other emergent vegetation that provides ample cover. In contrast, molting diving ducks such as canvasbacks, lesser scaup, and redheads prefer large bodies of open water, where they can dive to escape threats.

In addition to providing security, wetlands selected by molting waterfowl must also have sufficient food resources to meet the birds' nutritional demands. Molting is the second most nutritionally costly process in the annual cycle of waterfowl, just behind the formation of eggs. Waterfowl feathers are composed of 86 percent amino acids (the building blocks of proteins), a considerably higher concentration than is found in the invertebrates and plants that the birds eat. As a result, waterfowl must consume large quantities of protein-rich food while molting. For example, an adult mallard ingests approximately 102 grams of protein to replace about 66 grams of body and tail feathers shed during the molt. This means that a mallard must consume an average of about 3 grams of protein per day over a 30-day period, or about 31,000 medium-size invertebrates in total.

To meet the various biological requirements of the molt, many waterfowl undertake a "molt migration," which can span more than 800 miles. This migration occurs in summer, when large numbers of prairie-nesting waterfowl leave the breeding grounds and migrate to molting wetlands in the boreal forest and Arctic. Among ducks, drakes are the first to depart, sometime during incubation, often early in the process. Drakes typically are joined on molting wetlands by nonbreeding females and females whose early nesting attempts failed. Meanwhile, hens that successfully hatch a brood remain on the breeding grounds and molt on the same wetlands where they rear their young.

Tens of millions of waterfowl likely molt in the boreal forest and Arctic, and each summer huge concentrations of waterfowl occur on key molting wetlands in these northern regions. Often these wetlands will host a mix of molting and staging waterfowl, which are difficult to tell apart from one another but make for an impressive sight. For example, in late summer, up to 300,000 molting and staging lesser scaup have been observed on McCallum Lake in Saskatchewan, while Manitoba's Lake Winnipegosis annually hosts up to 80,000 molting redheads. Some molting wetlands also host large proportions of certain waterfowl species. A recent study discovered three previously unknown molting and staging wetlands in central Alberta that support between 5,000 and 7,000 Barrow's goldeneyes, or about 4 percent of the entire western North American population.

There are several hypotheses that may explain why waterfowl undertake molt migrations to the boreal forest and Arctic. One possibility is that the abundance of suitable, drought-resistant wetlands in the north provides molt migrants with greater access to food, which also tends to peak when these visitors arrive. In addition, with up to 24 hours of daylight in far northern latitudes, the birds have more time to forage, which might enable them to increase their daily nutritional intake.

Although waterfowl underwent molt migrations long before people arrived on the prairies and other southern breeding areas, many of the remote northern wetlands used by molting waterfowl remain pristine and largely undisturbed by people. Human disturbance can be extremely disruptive to flightless molting waterfowl, reducing foraging time, increasing energy expenditure, and potentially decreasing survival. As a result, we must ensure that land-use changes now occurring in the boreal forest and the Arctic do not negatively impact molting waterfowl and their fragile wetland habitats.

Mark Baschuk is a biologist and Dr. Stuart Slattery is a research scientist with the Institute for Wetland and Waterfowl Research at DU Canada headquarters at Oak Hammock Marsh.

Tell-Tail Feathers The natal down, characteristic of young waterfowl, is replaced by feathers weeks after hatching, depending on the species. When these first feathers form, some of the down may remain at the tips of the new feathers (far left). Often this downy tip will break off, forming a notched feather (center) that is commonly found in the tail feathers, or retrices, of birds. This characteristic can be used by biologists and hunters alike to determine a bird's age. A young-of-the-year bird will contain the downy tip or notch, whereas an adult bird will have fully formed feathers (right). These characteristic retrices of young birds are only present until late fall, when the juvenile retrices are molted and replaced with feathers lacking a notch.