The Trumpeter Swan (Cygnus buccinator) is the heaviest bird native to North America and is, on average, the largest extant waterfowl species on earth

These birds feed while swimming, sometimes up-ending or dabbling to reach submerged food. The diet is almost entirely aquatic plants.
They will eat both the leaves and stems of submerged and emergent vegetation. They will also dig into muddy substrate underwater to extract roots and tubers.
In winter, they may also eat grasses and grains in fields.