—Bruce Batt, Ph.D.
How long do pairs of waterfowl stay together? What happens if a mate is lost? Do all species do the same things while courting potential mates? These thoughts might come to mind when we see pairs of ducks and geese in spring. Waterfowl mating systems have been studied extensively around the world for many years, so there are answers to these questions for many species.
The two primary types of waterfowl mating systems are monogamy (having one mate) and polygamy (having several mates). Both occur in waterfowl, but monogamy is by far the dominant pattern. There is considerable variation in how each species is monogamous. Mallards and Canada geese offer good examples of different types of monogamy in waterfowl.
Mallards begin forming pair bonds in early winter, and by December, it's common to see pairs of mallards. Mate choice is ultimately up to the hen, but males choose which hens they court. In most duck populations, males outnumber females, which have higher mortality caused by predation during the nesting period. The limited supply of females and the resulting competition for mates are likely why male ducks developed elaborate plumage patterns and displays to help them attract the attention of females and perhaps discourage rival males.
Hen mallards lead their mate back to the breeding grounds, often to the same area where the hen was hatched and raised. Biologists presume that hens return to their natal breeding grounds because they are already familiar with important habitats like feeding areas and nesting cover. Male mallards have no such allegiance to a particular breeding area and may roam widely during their lifetime as they follow different hens back to the breeding grounds in successive years.
Once pairs arrive on the breeding grounds, the male's main job appears to be to defend his mate and her nesting territory from intrusion by other mallards. This is partly to provide the hen with undisturbed feeding time but also to defend the male's paternity. Unpaired males will vigorously attempt to mate with unguarded females, and despite the best efforts of paired drakes, genetic analysis has shown that many broods contain ducklings sired by different male parents. Thus, we can perhaps add promiscuity to the range of mating opportunities sought by male mallards—and females, too.
As nesting proceeds, the male becomes less and less attentive until he finally abandons his mate about the time she begins incubating. At this time, the drake will strike out in search of other breeding opportunities, leaving the female to hatch and raise her brood on her own. Meanwhile, the drake may pair up with another female that has lost her nest or mate. The propensity of mallards to form new pairs within a single breeding season can be thought of as a form of serial monogamy.
Most other ducks demonstrate similar patterns of monogamy, although pairing in diving ducks, for example, typically occurs during spring migration or after the birds have arrived on the breeding grounds. And while most ducks pair and nest as yearlings, many don't breed until their second or third year of life. Remarkably, some species such as Barrow's goldeneyes and buffleheads reestablish pair bonds on the wintering grounds with mates from previous years.
Canada geese are different. Pairs will typically remain together throughout the birds' lives. But this isn't absolute. In some cases, Canada geese will seek a new mate while their former mate is still alive. This typically occurs when one member of a pair is injured or becomes lost during migration or on the wintering grounds.
A significant difference between geese and ducks is that the goose family structure remains intact throughout fall migration, winter, and spring migration. The family breaks up only after the birds return to the breeding grounds and the adult pair begins to actively nest. At this time, the yearlings join flocks of other nonbreeding geese. When these young birds are two or three years old, they will find their own mates on the wintering grounds. The new pairs then follow the same adult life pattern described above. This is referred to as perennial monogamy whereas mallards demonstrate seasonal monogamy.
The other mating system known to waterfowl—polygamy—has been observed in only a few species. Forms of polygamy include polyandry (single females have several male mates) and polygyny (single males have multiple female mates). Polyandry has never been observed in waterfowl, but polygyny occurs in some species. In North America, the ruddy duck and muscovy typically do not form pairs at all. In these species, the male establishes a territory that he defends from other males but allows females to enter. Some males have the opportunity to breed with more than one female in their territory, thus illustrating one form of a polygynous mating system.
Waterfowl mating behavior varies considerably based on the birds' needs. For some ducks like the northern shoveler, defending a feeding territory is a dominant concern. For others, like the northern pintail, protecting hens from harassment by unpaired males may be critical. And for most geese, cooperation in all phases of family life is important to the birds' survival. So this spring, when you see a pair of ducks in a local wetland—perhaps mallards or blue-winged teal—see if you can figure out how the drake and hen are cooperating to maximize the success of the upcoming breeding season.
Mating Systems and Variation in Canada Geese
The family structure and mating systems of Canada geese are directly responsible for the existence of different races of these white-cheeked birds. When young geese form pairs of their own, they choose mates from flocks of young birds from the same areas where they were raised. Thus, the characteristics of their respective parents that were molded by local conditions are propagated in their own young—much like line breeding in dogs.
In this way, distinct localized populations have developed over thousands of generations, which we can see in the size, coloration, and conformation of each race. For example, southern giant Canada geese have large bodies, long necks, and generally light-colored plumage. In contrast, Canada geese that nest in the high eastern Arctic are much smaller, have shorter necks, and generally darker plumage. Since each population tends to return to its traditional breeding and wintering areas, biologists can manage the harvest of each population from these distinct locations separately.