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.