Mute swans pair for life and they mate and begin buiding a nest in March and April. The nest is built on the ground, near to water, in an undisturbed place.
The cob collects reeds and sticks, bringing them to the female so she can arrange them. The nest is often a very big platform-like structure, and may be the pair's old nest which has been rebuilt and used year after year. Although the cob and pen look very similar at first glance, they can be told apart by looking at their beaks. In the spring and summer the cob's bill is a brighter colour than the pen's, and the black knob is more bulbous. The cob is never far from his mate on the nest, keeping an eye out for intruders. If a potential predator gets too close, he will hiss at them (mute swans are quiet birds on the whole, but are not really mute!) and if necessary will charge at them with flapping wings.
The pen lays 5 - 8 large, greenish-brown eggs, one every two days. She does most of the incubation, which starts as soon as the last egg has been laid. This allows all the young to hatch at the same time, after 36 days. Soon after hatching, the young swans, called cygnets, covered with fluffy, grey down, leave the nest. Their parents pull up water plants for them to eat, and they snap up invertebrates (minibeasts) from the surface of the water.
The cygnets stay with their parents until the next winter by which time they are losing the brown plumage that replaced the grey down. It will be a full year before they are completely white, and they are ready to breed when they are three or four years old.Read More: Mute Swans and humans