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.