Like other swans, trumpeter swans often mate for life, and both parents participate in raising their young, but primarily the female incubates the eggs.
Most pair bonds are formed when swans are 5 to 7 years old, although some pairs do not form until they are nearly 20 years old. "
Divorces" have been known between birds, in which case the mates will be serially monogamous, with mates in differing breeding seasons.
Occasionally, if his mate dies, a male trumpeter swan may not pair again for the rest of his life.
Most egg laying occurs between late April and May. The female lays 3–12 eggs, with 4 to 6 being average, in a mound of plant material on a small island, a beaver or muskrat lodge, or a floating platform on a clump of emergent vegetation.
The same location may be used for several years and both members of the pair help build the nest.
The nest consists of a large, open bowl of grasses, sedges and various aquatic vegetation and have ranged in diameter from 1.2 to 3.6 m (3.9 to 11.8 ft), the latter after repeated uses.
The eggs average 73 millimetres (2.9 in) wide, 113.5 millimetres (4.5 in) long, and weigh about 320 grams (11.3 oz).
The eggs are quite possibly the largest of any flying bird alive today, in comparison they are about 20% larger in dimensions and mass than those of an Andean condor (Vultur gryphus), which attains similar average adult weights, and more than twice as heavy as those of kori bustards (Ardeotis kori).
The incubation period is 32 to 37 days, handled mainly by the female, although occasionally by the male as well.
The young are able to swim within two days and usually are capable of feeding themselves after, at most, two weeks.
The fledging stage is reached at roughly 3 to 4 months.
While nesting, trumpeter swans are territorial and harass other animals, including conspecifics, who enter the area of their nest.

Adults go through a summer moult when they temporarily lose their flight feathers.
The females become flightless shortly after the young hatch; the males go through this process about a month later when the females have completed their moult.
Like other swans, trumpeter swans often mate for life, and both parents participate in raising their young, but primarily the female incubates the eggs. Most pair bonds are formed when swans are 5 to 7 years old, although some pairs do not form until they are nearly 20 years old. "Divorces" have been known between birds, in which case the mates will be serially monogamous, with mates in differing breeding seasons. Occasionally, if his mate dies, a male trumpeter swan may not pair again for the rest of his life. Most egg laying occurs between late April and May. The female lays 3–12 eggs, with 4 to 6 being average, in a mound of plant material on a small island, a beaver or muskrat lodge, or a floating platform on a clump of emergent vegetation. The same location may be used for several years and both members of the pair help build the nest. The nest consists of a large, open bowl of grasses, sedges and various aquatic vegetation and have ranged in diameter from 1.2 to 3.6 m (3.9 to 11.8 ft), the latter after repeated uses. The eggs average 73 millimetres (2.9 in) wide, 113.5 millimetres (4.5 in) long, and weigh about 320 grams (11.3 oz). The eggs are quite possibly the largest of any flying bird alive today, in comparison they are about 20% larger in dimensions and mass than those of an Andean condor (Vultur gryphus), which attains similar average adult weights, and more than twice as heavy as those of kori bustards (Ardeotis kori). The incubation period is 32 to 37 days, handled mainly by the female, although occasionally by the male as well. The young are able to swim within two days and usually are capable of feeding themselves after, at most, two weeks. The fledging stage is reached at roughly 3 to 4 months. While nesting, trumpeter swans are territorial and harass other animals, including conspecifics, who enter the area of their nest.
Adults go through a summer moult when they temporarily lose their flight feathers. The females become flightless shortly after the young hatch; the males go through this process about a month later when the females have completed their moult.
Trumpeter swan