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.
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.
Drakes are quite strategic in their pursuit of EPCs. For example, drake lesser scaup and mallards attempt more EPCs in the morning, when the chance of fertilization is greatest. Male white-cheeked pintails use sophisticated tactics to achieve extrapair copulations, such as swimming in a submerged "submarine" posture up to a target female and her mate. Among snow geese, egg-laying is highly synchronous, which narrows the potential window for males to pursue EPCs. Male snow geese mate-guard their females until egg-laying ceases. Once the last egg is laid, the male may then pursue EPCs. Typically, male snow geese will pursue EPCs with neighboring females that already have a mate. Usually, the mate is absent when the invader initiates his EPC attempt. Often the EPC attempt occurs when the resident male is pursuing an EPC or is distracted by a territorial dispute with yet another male. Absent resident males return immediately when their mate is subjected to an EPC and will vigorously attack intruding males.
Waterfowl are unique among birds in that all EPCs appear to be resisted by females. This is true of almost every waterfowl species, with the possible exception of musk ducks and the muscovy. So why do female waterfowl always resist? One theory is that it's a strategy to test male fitness. However, hen wood ducks, mallards and Laysan ducks have all been observed drowning while resisting EPCs, so clearly the decision to resist is not trivial for hens.
In lesser snow geese at the La Perouse Bay colony in Manitoba, pair mating usually occurs on the water. In contrast, EPCs invariably occur on land. This is probably because it is much harder for females to escape on land; on the water, the female can dive.
In colonial geese, many EPCs occur on the nest itself. If the female resists too much on the nest, she risks breaking her eggs.
Prior to the advent of modern genetic techniques, it was very difficult to determine whether forced copulations resulted in fertilization. The first studies to document multiple paternity in captive ducks used genetically based feather coloration as an indirect genetic "marker." Researchers introduced males with alternative feather coloration to mated pairs. Because the feather coloration is genetically based, every duckling that exhibited the alternative coloration was obviously sired by the nonmated male. During the 1980s, analyses of proteins called allozymes provided a more direct measure of genetic composition. Allozyme studies brought into question the putative monogamy of several bird species. More recently, direct study of genetic material has made DNA fingerprinting possible using gene sequencing techniques.
Mating systems in birds are amazingly diverse, and they comprise a fruitful field of study for evolutionary ecologists. Waterfowl are no exception, and modern genetic techniques are helping behavioral ecologists determine what is really happening on the breeding grounds.