Swan
Behaviour

Swans feed in water and on land. They are almost entirely herbivorous, although they may eat small amounts of aquatic animals. In the water, food is obtained by up-ending or dabbling, and their diet is composed of the roots, tubers, stems and leaves of aquatic and submerged plants.

A familiar behaviour of swans is that they mate for life, and typically bond even before they reach sexual maturity. Trumpeter swans, for example, can live as long as 24 years and only start breeding at the age of 4–7, forming monogamous pair bonds as early as 20 months. "Divorce", though rare, does occur; one study of mute swans shows a 3% rate for pairs that breed successfully and 9% for pairs that do not. The pair bonds are maintained year-round, even in gregarious and migratory species like the tundra swan, which congregate in large flocks in the wintering grounds.

Swans' nests are on the ground near water and about a metre (3') across. Unlike many other ducks and geese, the male helps with the nest construction, and will also take turns incubating the eggs. Alongside the whistling ducks, swans are the only anatids that will do this. The average egg size (for the mute swan) is 113 × 74 mm (4 + 1 ⁄ 2 x 3 in), weighing 340 g (12 oz), in a clutch size of 4 to 7, and an incubation period of 34–45 days. Swans are highly protective of their nests. They will viciously attack anything that they perceive as a threat to their chicks, including humans. One man was suspected to have drowned in such an attack. Swans' intraspecific aggressive behaviour is shown more frequent than interspecific behaviour for food and shelter. The aggression with other species is shown more in tundra swans.
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