Ducks obtain energy by eating a mix of plants and small animals, then breaking that food down through a specialized digestive system that extracts nutrients for conversion into cellular fuel. The process spans from foraging in wetlands to molecular reactions inside muscle cells, and it shifts dramatically with the seasons, water temperature, and whether a duck spends its life on the surface or diving below it.
What Ducks Actually Eat
Ducks are omnivores with flexible diets shaped by what’s available. A duck foraging freely will eat green plants, insects, snails, frogs, seeds, and aquatic invertebrates. The balance between plant and animal food varies by species and season. Dabbling ducks (the ones that tip forward in shallow water) tend to eat more plant material, while diving ducks consume a higher proportion of animal prey like mollusks and crustaceans. Both types need a combination of carbohydrates, fats, and proteins to meet their energy demands.
Wetlands are the primary energy source for wild ducks. These habitats supply submerged vegetation, algae, insect larvae, and small fish in dense concentrations. The richness of a wetland directly determines how quickly a duck can build the fat stores it needs for migration and breeding.
How the Bill Works as a Filter
Many duck species don’t just grab individual food items. They filter-feed, pumping water through their bills to strain out edible particles. The edges of a duck’s bill are lined with comb-like structures called lamellae, thin ridges that act as a sieve. What’s remarkable is that ducks can actively adjust the spacing between these ridges by opening or closing their jaws slightly. This lets them selectively retain particles of a preferred size while flushing out debris or prey that’s too large.
There’s a trade-off built into this system: adjusting the jaw to be more selective about particle size slows down the rate of water filtration. So a duck choosing smaller, higher-quality food items processes water more slowly than one gulping everything. This mechanical trade-off helps explain why different duck species can coexist on the same pond, each targeting slightly different prey sizes.
Digestion: Acid Bath, Then Crushing
Once swallowed, food moves quickly to the first stomach chamber (the proventriculus), which drenches it in acid and digestive enzymes that begin breaking down proteins. From there, food passes into the gizzard, a thick-walled muscular organ that physically grinds and crushes it. Ducks swallow small stones and grit that sit inside the gizzard and act like millstones, pulverizing hard seeds, shells, and insect exoskeletons. A duck’s gizzard can generate internal pressures around 180 mmHg, strong enough to crack materials that would pass through a human stomach untouched.
This two-stage process, chemical then mechanical, is essential because ducks have no teeth. The gizzard compensates by doing the chewing internally, ensuring that tough food gets broken into particles small enough for the intestines to absorb nutrients efficiently.
Converting Food Into Cellular Fuel
At the cellular level, ducks produce energy the same way all birds and mammals do: through a process that breaks down sugars and fats inside mitochondria, the energy-producing structures in every cell. Nutrients from digested food enter the bloodstream and travel to tissues, especially muscles, where mitochondria use oxygen to convert them into ATP, the molecule that directly powers muscle contractions, body heat, and organ function.
Fats are a particularly important fuel source. Gram for gram, fat delivers more than twice the energy of carbohydrates, which makes it the ideal fuel for energy-intensive activities like flying and staying warm. Duck muscles are well-equipped to burn fat. Diving ducks, in particular, show elevated levels of enzymes dedicated to breaking down fatty acids for energy, reflecting how heavily they rely on lipid-based fuel during sustained underwater activity.
How Diving Ducks Power Their Dives
Diving ducks face a unique energy challenge: they need to power intense muscular work underwater while holding their breath. Their leg and chest muscles have adapted in measurable ways compared to dabbling ducks that stay on the surface. Diving species have significantly higher capacity for aerobic metabolism in their locomotory muscles, meaning they can sustain high rates of oxygen-dependent energy production during a dive. Sea ducks take this even further, with leg muscles showing elevated capacity for both overall energy production and fat burning.
Interestingly, diving ducks have not evolved a greater capacity for anaerobic metabolism (the emergency energy pathway that works without oxygen). Instead, they depend on oxygen stores in their blood and muscles to keep aerobic energy production running throughout a dive. This strategy is more efficient and avoids the buildup of lactic acid that would limit dive duration.
Diving also costs energy in another way. When a tufted duck submerges in cold water, roughly half the extra energy it spends compared to resting on the surface goes toward keeping its body warm. About half of that heat production happens after the dive, as the duck warms itself back up at the surface.
The Extreme Cost of Flight
Flying is by far the most energy-expensive thing a duck does. Studies on common eiders, a large sea duck, found that flight requires 16 to 20 times a duck’s resting metabolic rate. In raw terms, a flying eider burns energy at a rate of 123 to 149 watts, compared to about 7.5 watts at rest. That’s roughly the energy output of an incandescent light bulb, sustained entirely by muscle power.
This enormous cost is why ducks spend relatively little of their day in flight. Even during migration, they alternate between flying and extended stops at wetlands to refuel. The energetics of flight also explain why larger, heavier ducks face disproportionately higher costs. Their wings must beat faster relative to body size, and the power required increases steeply with mass.
Fat Storage Before Migration
To fund long migratory flights, ducks undergo a period of intense feeding called hyperphagia. During stopover periods at productive wetlands, ducks can gain weight rapidly. Canvasbacks in one study gained an average of 91 grams during feeding periods, with some individuals packing on up to 179 grams. That fat isn’t just insulation; it’s a fuel tank. The body breaks stored fat down into fatty acids that mitochondria in flight muscles burn during sustained travel.
The quality of stopover wetlands matters enormously. Ducks that land at nutrient-rich sites build fat faster and arrive at breeding grounds in better condition. Ducks that encounter degraded wetlands may not accumulate enough reserves to complete their migration or reproduce successfully. This is one reason wetland conservation has such a direct impact on duck populations.
Staying Warm Takes Serious Energy
Ducks spend much of their lives on or in cold water, and maintaining a core body temperature around 40°C (104°F) in that environment is metabolically expensive. Their bodies use several strategies to manage this cost. A countercurrent heat exchange system in the legs keeps warm arterial blood flowing close to cool venous blood returning from the feet, recapturing heat before it’s lost. Dense, waterproofed feathers trap an insulating layer of air against the skin.
When cold exposure is extreme, ducks can ramp up heat production in their muscles. Cold-acclimated ducklings increase the activity of specialized proteins in their muscle mitochondria that divert energy away from ATP production and release it directly as heat. This is essentially the same principle as shivering, but operating at a molecular level. Fatty acids play a dual role here: they serve as fuel for the heat-generating process and may also help activate the uncoupling proteins that make it possible.