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.