Swans, such as the common Mute swan, are large waterfowl that face considerable challenges maintaining their diet during the winter months. Freezing temperatures and the formation of ice drastically reduce the availability of their preferred aquatic food sources. The seasonal shift requires swans to adapt their foraging behavior and rely on less energy-dense foods to sustain their large body mass. This forces the birds to consume whatever plant matter remains accessible.
Primary Aquatic Vegetation
Swans primarily sustain themselves on submerged aquatic vegetation (SAV). This remains their main food source in winter as long as water remains open. They utilize their long necks to reach deep below the surface, a feeding method known as “up-ending,” where only their lower body and tail feathers remain visible. This allows them to exploit deeper water areas than most other dabbling waterfowl, accessing plants attached to the bottom sediment.
The majority of their winter diet consists of the residual or dormant parts of plants like pondweeds (including Sago and Horned Pondweed), eelgrass, and waterweed. They actively seek out the roots, tubers, and rhizomes of these plants, which store carbohydrates and nutrients to survive the cold season. While swans may consume up to eight pounds of vegetation daily, winter aquatic plants are often fibrous and lower in caloric value compared to new growth. The mechanical action of their feet also helps dislodge tougher, deeper-lying plant parts from the substrate, making them easier to ingest.
Foraging in Icy Conditions
When water bodies freeze over, submerged vegetation becomes inaccessible, forcing swans to shift their diet to whatever is available along the shore and on land. This change is a response to resource limitation rather than a preference for terrestrial food sources. They will graze on emergent vegetation, such as the stems and leaves of reeds or bulrushes that stand above the ice or snow line.
The birds also move into fields adjacent to the water, grazing on available grasses and searching for waste grain. Consuming residual corn, wheat, or barley provides a necessary boost of carbohydrates and energy for thermoregulation in cold temperatures. This terrestrial foraging is often less efficient than aquatic feeding, requiring them to spend more time actively seeking scattered food items. Swans rely on finding patches of open water to survive, as their ability to find food decreases significantly when ice cover is extensive.
The Role of Grit and Minerals in Winter Digestion
To process the tough, high-fiber vegetation that makes up the winter diet, swans require non-caloric materials known as grit. Grit consists of small, hard particles like sand, gravel, and pebbles, which the birds ingest deliberately or incidentally with their food. This material is held within the gizzard, a muscular part of the stomach, where it acts like a set of grinding stones.
The mechanical action of the gizzard walls contracting against the grit helps physically break down the dense plant matter, making nutrients more accessible for absorption. A lack of grit can impair the gizzard’s function, leading to the retention of fibrous material and affecting the rate of food passage. Swans also ingest sediment, which provides trace minerals, including calcium, necessary for maintaining bone health during periods when their primary plant diet is nutrient-poor.