Guillemots, like many other seabirds, begin to gather at their breeding sites, well before breeding actually begins, in late May. They congregate on rocks and cliff ledges, packed tightly together in large colonies; they need to be stimulated by a noisy crowd to mate successfully.
Courtship takes place in the water with one bird swimming around the other which spins to face it. They sometimes indulge in communal displays, several pairs circling and bobbing or standing up and flapping their wings.
The air around a guillemot colony is full of the birds' muttered growls of varying pitch. They are silent for the rest of the year when not breeding.
A single egg is laid directly onto the bare rock - no nest is made. The mottled egg is pear-shaped (pyriform), and this is a special adaptation so that the egg rolls round in a circle when disturbed rather than off the ledge.
The male and female birds take turns to incubate the egg, balancing it on their feet, covering it with their belly plumage. After about 4 weeks, a grey, almost helpless youngster hatches and it will stay on its ledge for four to five weeks. The parents continually fly in and out from the colony to catch fish and regurgitate meals for their baby.
The young guillemots leave their ledges before their flight feathers have fully grown and they are only one-third of the adult weight. They leap off the ledges at night, to avoid predators such as gulls, and plummet down, whirring their tiny, undeveloped wings. On hitting the water they call frantically to their parents who are already at sea. Their parents escort them further out to sea where they are fed for about three weeks until they can fly and fend for themselves. The young birds will not return to land until two or more years later when they are ready to breed.
At the end of the breeding season, about the end of July, all the activity on the rock ledges ceases as the adults fly away southwards and seawards for the winter. The birds winter off southern England, France and Spain. Shortly after leaving the breeding grounds, the guillemots moult, shedding all their flight feathers at once and they cannot fly until a new set grows.Read More: Guillemots and Humans