Northern Prairie Wildlife Research Center
Mallards nested throughout Iowa prior to European settlement, but were most abundant in the seven-million acre prairie-pothole region that stretches from Des Moines to Spirit Lake to Mason City. As farming practices eliminated more than 98 percent of this wetland and grassland habitat, mallard populations and nest success declined dramatically.
Mallard pairs begin arriving in Iowa between late February and mid March and select breeding sites that can include several ponds in a square mile area. Many hens home back to the general area where they first learned to fly, and occasionally to the same field or nest structure where they previously hatched a clutch.
Although mallards eat waste grain and weed seeds during spring migration, upon arriving at a breeding site, the hens switch to feeding on invertebrates (primarily insect larvae and snails) in shallow ponds or bays (less than one foot deep). These invertebrates supply the protein and calcium hens need to produce eggs.
Most mallards nest at 1 year of age. Hens usually begin nesting between late March and May 1. They prefer to nest in grass fields where the residual vegetation is more than 1 foot tall and dense enough to provide overhead cover (Figure 7). Although mallards may nest side-by-side, nests are usually scattered throughout fields at densities ranging from 1 to 8 nests per 40 acres. Mallards will also nest over water on muskrat houses and clumps of cattails if they are available.
Hens lay one 2 1/4-inch long, beige to greenish buff egg per day. Average clutch size is 9 eggs but completed clutches can range from 1 to 18 eggs. Incubation begins the day the last egg is laid and averages 28 days. Drakes abandon their hens during incubation, leaving the hens to raise the young by themselves.
Mallard nests are frequently destroyed by farming operations (especially hay mowing) and predators (skunks, raccoons, fox, crows, and ground squirrels). If a nest is destroyed, the hen often attempts to nest again. She may make repeated attempts to hatch a clutch, even into July.
The hen and ducklings leave the nest together within 12 hours of hatching and take up residence on nearby wetlands. Preferred brood rearing wetlands have high numbers of aquatic insects for the ducklings to eat and emergent vegetation (cattails, bulrushes) for escape cover from predators. The family may move overland several times to find wetlands with suitable food and cover. Ducklings attain flight when 7 to 9 weeks old. About half the brood survives to flight stage.
After leaving their hens, drakes gather in flocks and migrate to large marshes to molt. Hens begin molting on brood rearing areas or nearby wetlands after the young can fly. Adult mallards are flightless for 4 to 5 weeks while growing new flight feathers. Family groups do not stay together during migration and hens find new mates each year on the wintering areas.
Because mallards occasionally nest on cattail clumps over water, hens can often be enticed into using over-water nest structures. However, unlike goose nesting structures, nest structures for mallards must have some lateral and/or overhead cover (Figure 7). And because mallards can not effectively defend their nests, nest structures must be placed over water far enough from shore that they are not accessible to predators.
When deciding which nest structure to use, select one that fits your budget, is aesthetically pleasing, is appropriate for your wetland and can be easily maintained. Maintenance is the key to consistent, successful use.