By J. Jasper Lament, Ph.D.
Resplendent in their breeding plumage, mated pairs of ducks disperse across nesting grounds in early spring. Attentive drakes appear to stand guard while females load up on protein for the big egg-production task ahead. As they patrol their territory, aggressively chasing away interloping pairs, it’s tempting to anthropomorphize and assume that a happy and loyal marriage has been attained. But it’s not nearly as simple as that.
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Duck mating systems can be rather complex and even chaotic. It is well known that drakes of most species contribute little or nothing to parental care. But it’s also true that a given duck brood might contain offspring from several different males. Any duckling not fathered by its mother’s mate is an example of what waterfowl scientists call “extrapair paternity.” A study of mallards in Manitoba revealed that 17 to 25 percent of clutches of eggs examined were the product of more than two parents. A gadwall study in North Dakota found that 11 of 261 ducklings (4.2 percent) within 8 of 29 broods (27.6 percent) had genotypes consistent with extrapair fertilizations.
In contrast to the more promiscuous ducks, geese and swans are sometimes held up as an example of lifelong monogamy, with two committed parents mating for life. Monogamy happens to be a rather rare mating strategy in the animal kingdom, and this is certainly the case in some geese as well. Researchers have demonstrated that some goose broods contain offspring from two or more males. Females that lay eggs in nests other than their own (known as brood parasites) often mate with resident males. This behavior has been recorded in the barnacle goose.
Extrapair paternity is the result of extrapair copulations (EPCs). EPCs are commonplace in most bird species. In some songbirds, it appears that females may actively solicit EPCs as a way to get higher quality fathers for some of their offspring. One study of mallards estimated that at least half of females were involved in multiple mating events. The authors wrote that males appear to “compromise between forced copulation and minimizing the likelihood of being cuckolded.”
As many hunters have observed, adult sex ratios in dabbling ducks are typically skewed, with significantly more males available than females. This means that many males will not obtain a mate. Early researchers speculated that it was unpaired males that sought out EPCs. But it’s now clear that in most cases it’s paired males that pursue the vast majority of EPCs. Furthermore, the main strategy of unpaired males is to establish a pair bond with a female, not to seek out EPCs. Thus, EPCs are an attempt by mated males to increase their reproductive success, not an alternative strategy pursued by some males. There are a few exceptions; in wood ducks, forced copulations are pursued (albeit rarely) by unmated males.
Paired drakes of almost every dabbling duck species attempt forced EPCs. The list includes northern pintails, black ducks, shovelers, blue-winged teal, cinnamon teal, American wigeon, green-winged teal, gadwalls, wood ducks and mallards. In contrast, EPCs are uncommon in mottled ducks, and most are attempted by males whose mates have already hatched a nest or are still incubating.