Mallard pair mounting in the autumnIn the case of the Mallard, our commonest duck, there is social monogamy, with pairing observed at least for six months of the year and lasting at least several years. Pairing starts after the moult in the autumn, when even pair displays and mounting are common, even though breeding cannot occur. Males do not share the parental duties, the female being in charge of incubating, brooding, leading the ducklings and keeping them out of danger. Pairs can be seen carrying out a ritualised courtship, but a different mating strategy is commonly seen, in which one or several males chase a female and force her to copulate.
Group of drakes attempting copulation with a femaleFemales do try to escape these attempts by flying, diving or hiding and call loudly - presumably for their partner - and they are known to have been injured or even drowned in the process, probably due to the number of males trying to hold onto the females neck to secure copulation. In most cases females are not injured - other than losing some neck feathers - and forced copulations can result in fathered offspring, so it is in the interest of the drake to guard the female against this 'extra-pair' copulations to ensure that he fathers as much offspring as possible. Drakes maintain very close proximity to their mates during a critical period, - up to four days before the beginning of egg laying and up to four days afterwards - following the females everywhere. Males only swam away from their partners to chase other males away or initiate fights when other males approached his partner. Males also attempted to stop forced copulations on their partner with varied success. But it was also during the critical period that both paired and unpaired males initiated approaches to paired females. If the female was on its own, she called loudly if other males approached. Mate guarding also benefits females allowing them to forage more efficiently and possibly, decreasing their mortality due to predation because of their mate's alert state. Males with high testosterone levels in spring are more efficient guarding their females, and females show a preference to pair with such males during the autumn.
Cunningham, Emma J. A. (2003) Female mate preferences and subsequent resistance to copulation in the mallard. Behav. Ecol. 14: 326-333. here.
Davis, Ellen S. (2002) Female choice and the benefits of mate guarding by male mallards. Animal Behaviour, 64: 619-628. here.
Goodburn, S. F. (1994) Mate Guarding in the Mallard Anas platyrhynchos. Ornis Scandinavica 15: 261-265. here.