Breeding
Thick-billed Murres form vast breeding colonies, sometimes composed of over 1 million breeding birds, on narrow ledges and steep cliffs which face the water. They have the smallest territory of any bird, requiring less than one square foot per individual. A breeding pair will lay a single egg each year. Despite this, they are one of the most abundant marine birds in the Northern Hemisphere.
Adults perform communal displays early in the breeding season to time their breeding cycles. They do not build nests, but lay the egg directly on bare rock. Both parents are involved in incubating the egg and raising the young. Due to the enormous amount of energy needed to take off in flight, adults can only provide one food item at a time to their chick. Chicks spend between 18 to 25 days on the cliffs before leaving for the sea. Once ready to leave, the young will await nightfall and jump off the edge towards the water. A parent immediately jumps after and glides within centimeters of the fledgling . At sea, the male and the chick stay together for around 8 weeks during which the adult continues to provide food for the young
Survival rates of the young is not based on the number of individuals in the colony, but rather on the age of the breeders within the colony. Offspring of inexperienced pairs grow more slowly than those of experienced breeders, possibly because they do not receive as much food from their parents. Also, pairs which contain at least one young breeding bird tend to have lower hatch rates. Older and experienced adults obtain the better nesting sites located in the center of the colony, while the inexperienced individuals are kept on the margins where their young are more likely to be preyed upon.
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