The adult birds return to the colonies in late March or early April, and initially spend a long time on the sea in large flocks called rafts.
Where possible, the birds excavate a nesting burrow into the soil. Sometimes they will make use of Manx shearwater or rabbit burrows. Where burrowing is not possible, the birds nest under boulders or in cracks and cavities in cliffs.
The birds defend the nesting site and its immediate surround, and use it in subsequent years. Puffins lay only a single egg, in late April or early May. Both parents incubate it for 36-45 days, and they share the feeding duties until the chick is ready to fledge.
The fledging period is very variable, ranging from 34 to 60 days, depending on the area and year.
Adult birds desert their young shortly before they are ready to leave the nest. The timing of the breeding in puffin colonies is highly synchronised, and so the departure of all adults takes place within a few days.
The young birds leave their nest burrow and make their way to the sea, normally under cover of darkness to avoid predators. In some colonies, for instance in Iceland, nearby bright lights confuse the young birds, which then fly into the light and end up on city streets.
Puffins usually reach breeding age at 5-6 years old, and often live for 20 years.