Foraging distribution has been shown to change with tidal cycles where birds prefer waters of moderate flow. Reproduction Black guillemots breed in relatively small scattered colonies and lay 2 eggs. The typical reproductive cycle is as follows. More experienced parents often lay slightly earlier and have a larger mean clutch size. Once the last egg is laid incubation is continuous with both parents sharing shifts for 28 to 32 days.
Colony attendance is highest in the early morning.
As the chicks get older they wander inside the nest crevice. Both sexes feed the demanding chicks up to 20 fish a day until they fledge at age 30 to 40 days. Sometimes parents must entice young from the nest with fish, but once fledged, chicks are on their own. The average breeding success ranges from 0.48 to 1.6 young per pair with losses due to predation, bad weather, and flooding from high tides. Behavior Black guillemots form monogamous pairs and show remarkable site fidelity, returning to the same colonies and often the same nest site year after year.