—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.