Like many animals, waterfowl form pair bonds with members of the opposite sex for the purpose of reproducing. The types of pair bonds formed, however, are not what one would think. Waterfowl mating systems vary considerably. Some species pair for life, whereas others invest a lot of time forming new pair bonds each year—an activity that at first glance would seem too costly and time consuming.

Only about 44 percent of waterfowl species—all of which are geese and swans—form long-term, monogamous bonds. That means that the males of the remaining species must form new bonds each year by finding a new mate, investing in courtship displays and competing with other males.

Monogamy, or pairing for life, is common in geese and swans. They do not form bonds until they are at least two years of age, but more commonly do so in their third or fourth year of life. Therefore, geese do not nest and lay eggs until their second year or later, and swans typically do not begin laying until their fourth year. Male geese play a significant role in raising young, including vigilance over and defense of females while they are incubating and brood rearing. If one of the pair dies, the other will eventually re-pair, but this may interfere with or prevent the surviving mate from breeding for that year. Divorce has also been noted in geese, in which pairs will separate. Divorce has been seen in pairs that were not successful in their nesting attempt or in laying and hatching eggs.

Either way, there is a cost of failed breeding in a year if a mate is lost.

Ducks do not form long-term pair bonds, but instead form seasonal bonds, otherwise known as seasonal monogamy, in which new bonds are formed each season. Seasonal monogamy occurs in about 49 percent of all waterfowl species. In this mating system, pairs generally form on the wintering grounds in the first year of life, and those bonds are maintained only through egg laying. Each winter, the birds must find a new mate and establish a new bond for that breeding season. Males that form seasonal bonds do not participate in raising the young, but will defend the space around mated females to prevent other males from gaining access to their mate. If the male of the pair dies during spring migration north, females will quickly find a new mate for that season, and nesting will not be delayed in that year. Seasonal monogamy is common among dabbling ducks, diving ducks and some sea ducks.

An interesting twist on seasonal monogamy occurs in some cavity nesters and sea ducks that do not form bonds until their second year of life. Research has shown that some goldeneye pairs reunite each year on the wintering grounds and return to their previous breeding territory. This system is possible only in species that exhibit strong philopatry to both wintering and breeding sites. Philopatry is a behavior in which individuals return to the exact site, either on the breeding or wintering ground, from the previous year, enabling pairs to find each other. Males do not participate in raising the young, but they do defend females. Re-pairing is also suspected for buffleheads, long-tailed ducks, harlequin ducks and common eiders.

The final mating system observed in waterfowl is polygamy, in which multiple partners can occur. Polygamy is uncommon among waterfowl and observed in only 7 percent of species, including the ruddy duck, musk duck (Australia), comb duck (South America, Africa and southern Asia), and maccoa duck (Africa), all of which are stiff-tail ducks, and the magpie goose (Australia). In this system, pair bonds are weak or not formed at all, but instead males defend mating territories that may attract several females. For example, male musk ducks establish and defend breeding territories along shorelines and engage in elaborate courtship displays to attract females to their territories. Females visit these territories, and the males will mate with several females. In North America, the ruddy duck is the only duck to occasionally exhibit polygamy (they also form seasonally monogamous pair bonds). The polygamous mating system of waterfowl is not well studied or understood.

Why do such differences exist? That question is difficult to answer and falls into the category of "which came first, the chicken or the egg?" There are many theories as to why different mating systems evolved; researchers, however, will never truly be able to determine the evolutionary factors that shaped mating systems. Instead, there are several characteristics of waterfowl species associated more often with the different mating systems. Long-term pair bonds are generally observed among species of waterfowl that have large bodies; live longer because of lower annual mortality; exhibit low annual production (fewer young produced); have slow-maturing young; exhibit high philopatry to the breeding and wintering sites and depend on limited food resources on the breeding grounds.

These characteristics are typical of geese and swans. On the other hand, seasonal pair bonds are more typical of species with small bodies; species that exhibit higher annual mortality; higher annual productivity and breed in seasonal, highly productive environments. These characteristics apply to most dabbling ducks and diving ducks, such as mallards, teal, canvasbacks and redheads, to name a few.