The Black Guillemot usually nests singly or in very loose colonies, unlike the densely packed colonies of the Common Guillemot and Razorbill. The nest is placed out of sight in a rock crevice, beneath a boulder or in a fissure in the rock close to the water. The female normally lays 2 eggs, which also sets the species apart from Sweden's other auks, which lay only a single egg. Both parents incubate for about four weeks and then feed the young in the nest until they are ready to fledge.