Mallards are very adaptable and use a broad range of habitats and foods, depending on the season.  Their varied diet of native wetland plants, agriculture plants, and animal matter (e.g., aquatic invertebrates) ensures that they receive the nutrition necessary for survival.  Native plants can be further subdivided into categories of seeds, tubers, bulbs and rhizomes, and leafy vegetation.  Native plants tend to be lower in carbohydrates than agricultural grains.  Leafy aquatic vegetation tends to be the poorest food source because of its high water and fiber content.  Animal matter provides the highest source of protein and a nearly equal energy value from carbohydrates when compared to plants.

Winter feeding is a complex interaction of food availability, nutritional needs, habitat quality, and waterfowl behavior.  In general, wintering mallards consume 43% to 48% wetland plants, 49% to 54% agricultural grain, and approximately 3% animal matter.  However, these figures can vary dramatically.  The degree to which mallards feed on agricultural grains is affected by the abundance and availability of native foods.  Amount of rainfall can also affect where and how waterfowl feed. 

Fall and winter foods consist primarily of high-energy seeds from aquatic and cultivated plants.  In the spring, females eat aquatic invertebrates for egg production, while males continue eating primarily seeds and plants.  During the molt, males shift their diet to invertebrates for nutrients supporting feather growth.  In the summer, adults and ducklings eat invertebrates, but shift to high-energy seeds and plants as the fall migration approaches.

Ducks apportion their feeding based on energy received from various foods.  Seemingly, ducks assess the nutritional value of these foods compared to their abundance and availability.  For example, a large biomass of native seeds can be produced in an impoundment for ducks, but if the water is too deep, or there is too much downed wood or other debris covering seeds on the bottom, or the water is too muddy or iced over, the ducks may be unable to feed on these seeds. 

So having abundant food is not enough.  Other factors such as food availability, nutritional value, habitat quality, rainfall, and proximity to other foods are believed to affect the distribution and abundance of mallards.