Mute swans nest on large mounds that they build with waterside vegetation in shallow water on islands in the middle or at the very edge of a lake. They are monogamous and often reuse the same nest each year, restoring or rebuilding it as needed. Male and female swans share the care of the nest, and once the cygnets are fledged it is not uncommon to see whole families looking for food. They feed on a wide range of vegetation, both submerged aquatic plants which they reach with their long necks, and by grazing on land. The food commonly includes agricultural crop plants such as oilseed rape and wheat, and feeding flocks in the winter may cause significant crop damage, often as much through trampling with their large webbed feet, as through direct consumption. It will also feed on small proportions of aquatic insects, fish and frogs.
Unlike black swans, mute swans are usually strongly territorial with just a single pair on smaller lakes, though in a few locations where a large area of suitable feeding habitat is found, they can be colonial. The largest colonies have over 100 pairs, such as at the colony at Abbotsbury Swannery in southern England, and at the southern tip of Öland Island, Ottenby Preserve, in the coastal waters of the Baltic Sea, and can have nests spaced as little as 2 m (7 ft) apart. Non-mated juveniles up to 3–4 years old commonly form larger flocks, which can total several hundred birds, often at regular traditional sites. A notable flock of non-breeding birds is found on the River Tweed estuary at Berwick-upon-Tweed in northeastern England, with a maximum count of 787 birds. A large population exists near the Swan Lifeline Station in Windsor and lives on the Thames in the shadow of Windsor Castle. Once the adults are mated they seek out their territories and often live close to ducks and gulls, which may take advantage of the swan's ability to reach deep water weeds, which tend to spread out on the water surface.
The mute swan is less vocal than the noisy whooper and Bewick's swans; they do, however, make a variety of sounds, often described as "grunting, hoarse whistling, and snorting noises." During a courtship display, mute swans utter a rhythmic song. The song helps synchronize the movements of their heads and necks. It could technically be employed to distinguish a bonded couple from two dating swans, as the rhythm of the song typically fails to match the pace of the head movements of two dating swans. Mute swans usually hiss at competitors or intruders trying to enter their territory. The most familiar sound associated with mute swans is the vibrant throbbing of the wings in flight which is unique to the species and can be heard from a range of 1 to 2 km (0.6 to 1 mi), indicating its value as a contact sound between birds in flight. Cygnets are especially vocal and communicate through a variety of whistling and chirping sounds when content, as well as a harsh squawking noise when distressed or lost.
Mute swans can be very aggressive in defence of their nests and are highly protective of their mate and offspring. Most defensive acts from a mute swan begin with a loud hiss and, if this is not sufficient to drive off the predator or intruder, are followed by a physical attack. Swans attack by striking at the threat with bony spurs in their wings, accompanied by biting with their large bill, while smaller waterbirds such as ducks are normally grabbed with the swan's bill and dragged or thrown clear of the swan and its offspring. Swans will kill intruders into their territory, both other swans, and geese and ducks, by drowning, climbing onto and pecking the back of the head and forcing the other bird underwater.
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