Mallards usually form pairs in the fall, until the female lays eggs at the start of nesting season which is around the beginning of spring. Then she is left by the male who joins up with other males to await the moulting period which begins in June.
During the brief time before this, however, the males are still sexually potent and some of them either remain on standby to sire replacement clutches or forcibly mate with females that appear to be isolated or unattached regardless of their species and whether or not they have a brood of ducklings.
The nesting period can be very stressful for the female since she lays more than half her body weight in eggs. She requires a lot of rest and a feeding/loafing area that is safe from predators.
This can include nesting sites in urban areas such as roof gardens, enclosed courtyards, and flower boxes on window ledges more than one story up, which the ducklings cannot leave safely without human intervention. The clutch is 8–13 eggs, which are incubated for 27–28 days to hatching with 50–60 days to fledgling.
The ducklings are mature and fully capable of swimming as soon as they hatch. However, filial imprinting compels them to instinctively stay near the mother not only for warmth and protection but also to learn about and remember their habitat as well as how and where to forage for food.
When ducklings mature into flight-capable juveniles, they learn about and remember their traditional migratory routes. After this, the juveniles and the mother may either part or remain together until the breeding season arrives.
Mallards are opportunistically targeted by brood parasites, occasionally having eggs laid in their nests by Redheads, Ruddy Ducks, Lesser Scaup, Gadwalls, Northern Shovelers, Northern Pintails, Cinnamon Teal, Common Goldeneyes, and other Mallards. These eggs are generally accepted when they resemble the eggs of the host Mallard, although the hen may attempt to eject them or even abandon the nest if parasitism occurs during egg laying.